The United States and the Soviet Union, in a joint statement on the Persian Gulf War last night, told Iraq that hostilities could end "if Iraq would make an unequivocal commitment" to pull out of Kuwait and take "immediate, concrete steps" leading to compliance with the United Nations resolutions.

The statement, issued by Secretary of State James A. Baker III and Soviet Foreign Minister Alexander Bessmertnykh, also called for a redoubled effort after the war to resolve broader Middle East issues, including the regional arms race and the Arab-Israeli conflict.

Using expansive language in describing the goals of the two nations for the Middle East, the statement called for a "comprehensive settlement," but stopped short of specifying how this should be carried out. It included no mention of an international peace conference, which the Soviets actively have sought in the past.

The statement represented the first attempt by the United States and Soviet Union to spell out how Iraqi President Saddam Hussein might end the war: an ironclad promise to abandon Kuwait and a concerted effort to do so. Although the two foreign ministers reiterated their commitment to U.N. resolutions seeking the liberation of Kuwait, the statement appeared to mark a shift in tone from President Bush's adamant declaration as recently as Jan. 23 that there could be "no pause now that Saddam has forced the world into war." Bush vowed then, "We will stay the course and . . . succeed, all the way."

Administration officials said that neither Washington nor Moscow expected Saddam to act on the proposal. But they said the statement served the purpose of calming nervous Arab allies and quelling speculation that Soviet President Mikhail Gorbachev was straying from the international coalition. Last weekend, Bessmertnykh questioned whether allied military forces, with their extensive air bombardment of Iraq, were expanding the conflict beyond the U.N. goal of freeing Kuwait.

The statement also was significant because, after months of insisting that broader Middle East issues could not be linked to the gulf conflict, the superpowers addressed both in the same document. Both nations signaled they were prepared to move quickly after the war on some of the most complicated and nettlesome regional disputes, including the Palestinians, arms control and reconciliation between Israel and Arab states.

The joint communique was issued on the 13th day of the war, as public attention focused on the continuing departure of Iraqi warplanes to Iran. U.S. military officials said allied aircraft intend to destroy Iraqi jets attempting to enter or leave Iranian sanctuaries, but the exodus is considered more a symptom of Baghdad's desperation than a military threat.

Fair weather and the apparent disintegration of organized Iraqi air defenses allowed hundreds of allied bombers to criss-cross southern Iraq and Kuwait with virtual impunity. In one of the most successful assaults to date against Iraqi ground forces, Marine Harrier jets and other allied warplanes swooped down on an Iraqi convoy near the Kuwaiti-Saudi border, destroying 24 tanks, armored personnel carriers and trucks, U.S. military officials said.

Targeting this week has focused with increasing intensity on Republican Guard units deployed in southern Iraq and northern Kuwait, who are viewed by U.S. strategists as the key to breaking the back of the Iraqi military.

Several dozen B-52 bombers systematically have hammered the Republican Guard round the clock. Two days ago, U.S. intelligence detected the first "minor movement" by some of those units, a possible sign that the bombing could be forcing the troops to disperse for survival, Defense Department officials said yesterday.

"The Republican Guard have been well protected by their antiaircraft artillery, but this protection has gotten progressively thinner as our Jaguars have bombed these positions day after day," Group Capt. Niall Irving of Britain's Royal Air Force said yesterday in Riyadh. Guard "ammunition dumps, artillery and communications have been constantly attacked since day one, preparing the way for the possible land offensive," Irving added.

Iraq announced yesterday that one of the downed allied pilots it holds was killed in an air raid Monday night on the Ministry of Industry in Baghdad, according to an Iraqi News Agency account. Iraq, which says it holds more than 20 allied prisoners of war, warned last week that it would move them to potential target sites; on Monday, an Iraqi broadcast said an unspecified number of POWs had been wounded in raids. No additional details were provided.

The United States had no confirmation of the Iraqi report, White House press secretary Marlin Fitzwater said, adding that the report "clearly indicates that they're using POWs for human shield kinds of purpose . . . which, of course, is a direct violation of the Geneva Conventions."

State Department spokesman Margaret Tutwiler denounced as "barbaric" the practice of using prisoners as "human shields." The department summoned Khalid J. Shewayish, the senior Iraqi diplomat in Washington, to protest.

Allied warplanes flew more than 2,600 missions yesterday -- more than half of them bombing runs -- bringing to more than 27,000 the number of sorties flown since the war began two weeks ago. No U.S. aircraft were reported lost yesterday, and a U.S. F-15 Eagle shot down an Iraqi MiG-23. The Iraqis launched no Scud missiles toward Israel or Saudi Arabia.Patrol Boats Attacked

Saudi Arabian troops surprised an Iraqi artillery company preparing to launch an attack using short-range Frog missiles, Saudi Col. Ahmed Robayan told reporters in Riyadh. "We effectively shot the enemy down" with 16 surface-to-surface rockets, Robayan said.

British Lynx helicopters, flying patrols from HMS Brazen and HMS Gloucester in the northern Persian Gulf yesterday, spotted 17 Iraqi patrol boats, armed with machine guns, hugging the Kuwaiti coast. The helicopters attacked at 4 p.m. Saudi time (8 a.m. EST) with missiles after summoning reinforcements from American and Saudi helicopter gunships. A British officer reported the flotilla to "be scattering and taking refuge in a small bay," and under attack by allied fighter-bombers.

A senior U.S. military officer yesterday said that the United States and its allies already have scored impressive successes in knocking out a number of bridges that Iraqi supply trucks must use to replenish troops entrenched in Kuwait and southern Iraq. The trucks have to cross about a dozen major bridges spanning the Tigris and Euphrates rivers, some of which withstood earlier bombing, to reach Kuwait from Baghdad. Some structures comparable to Washington's 14th Street Bridge over the Potomac have now been destroyed, the officer said, largely because of the precise destructive power of "smart" guided bombs.

A senior Arab diplomat here said that for the most part the allies "are leaving alone the regular {Iraqi} army," which is "really in terrible shape," in order to concentrate on the Republican Guard and exploit the traditional rivalry between elite guard units and regular troops.

"Our hope is that if we can damage the Republican Guard enough, we really could strike a deal with the front-line generals," the diplomat said. Asserting that allied forces and Iraqi troops have maintained regular communications despite the war, the diplomat said he envisions Iraqi generals reaching a point of either surrendering or, more likely, rebelling against Saddam.

The massive oil spill blamed by U.S. officials on Iraqi sabotage of a Kuwaiti oil terminal continued to drift southward, threatening the coastlines of Saudi Arabia and possibly Iran. Officials in Saudi Arabia confirmed yesterday that a U.S. attack on Kuwaiti pipeline facilities last weekend succeeded in throttling the flow of crude oil into the spill, which now measures 60 miles by 20 miles and contains an estimated 11 million barrels. The southern edge of the slick is about 25 miles below the Saudi-Kuwaiti border and "appears to be breaking up," Army Brig. Gen. Pat Stevens IV said in Riyadh.

Easterly winds nudged the black lake away from the Saudi coast for now, but an oil spill computer model developed by Applied Science Associates Inc., a Rhode Island firm, projected that the oil would reach the world's largest desalination plant, in Jubail, by midday Saturday, and the coast of Qatar by mid-February.

A team of experts from the U.S. Environmental Protection Agency, the National Oceanic and Atmospheric Administration and other agenices flew over the slick yesterday as a Norwegian pollution-battling ship that was used to help fight the Exxon Valdez spill in Alaska arrived on the scene.

Booms have been placed around several small gulf islands to protect vital nesting grounds for thousands of birds and sea turtles, but industry experts warned that high waves or heavy currents could splash oil over the barriers. A Saudi official said his country has been inundated with offers of help "from all directions," including 70 tons of oil booms, suction skimmers and other equipment being flown in from the Oil Spill Response Center in Southampton, England.

The White House reacted harshly to Saddam's interview with Cable News Network (CNN) correspondent Peter Arnett, saying the Iraqi leader's comments threatening the use of such nonconventional weapons as chemical and biological warheads demonstrated "the amoral nature" of his tactics. "Here's a man who clearly is capable of using and willing to use and, indeed, brags about the prospect of using weapons of mass destruction," Fitzwater said.

A senior Arab diplomat in Washington voiced concern about Saddam's possible use of such weapons. "Chemicals. That's really the biggest surprise we worry about. . . . If he uses chemicals or biologicals, I think all bets are off," the diplomat said. "Really, I can't even begin to tell you what will happen then because we will all go back to the drawing board. . . . Every restriction we have had before will be thrown out the window."

More intensive bombing of Baghdad -- possibly including antiaircraft sites atop civilian buildings -- could be part of the allied response to chemical warfare, the diplomat added. Although many analysts believe Saddam will not use chemical weapons until he is desperate, "I personally think he will use them from the word 'Go' {once the ground war begins}," the Arab diplomat said.Soviets Proposed Language

Asked about Saddam's assertion in the CNN interview that he is certain of Iraqi victory, Baker said, "I would read that as whistling past the graveyard."

In Baker's talks with Soviet officials in recent days, administration officials said the Soviets said they would not agree to a cease-fire that would undercut the U.N. resolutions. But, they said, it was the Soviets who proposed the language offering to cease hostilities if Saddam provided a commitment to pull out, and some "immediate, concrete steps" leading to total withdrawal.

U.S. officials believe that Moscow, looking ahead to the end of the war, is trying to preserve its traditional ties to the Arab world by trying to appear to be a broker for peace.

Several officials said the message was that the allies would not continue to pursue Iraq militarily if it began a total retreat. One official said the statement offers Iraq "a way out of the conflict if they declare that they are going to implement the U.N. resolutions in toto."

"Obviously, we're not going to pound them continuously if they've declared they are getting out and have begun getting out," the U.S. official said.

But even this offer was carefully hedged. The statement said a halt in hostilities "would be possible" if Iraq fulfilled these conditions. Said another senior official, "That doesn't mean it is inevitable."

The communique also reflected Soviet concerns, following 13 days of allied air bombardment of Iraq, that the war not be carried beyond the U.N. resolutions calling for Iraqi withdrawal from Kuwait. Both nations "seek to avoid further escalation of the war and expansion of its scale," the statement said.

Coming just hours before Bush's State of the Union address last night, the statement seemed to catch some U.S. officials by surprise. Fitzwater said Bush was shown a copy after it was drafted and just before his address. The spokesman denied that the statement represented any change in the U.S. position on conditions for Iraq to pull out of Kuwait.

"We can now say that we made {Iraq} the offer" of a cessation of hostilities even after the Jan. 15 deadline set by the U.N. Security Council for Iraqi withdrawal, a U.S. official said. But "nobody, including the Soviets, believes they'll take it up."Success Against Scuds

On the broader Middle East goals, the communique said Washington and Moscow would make it a "high priority" after the war to achieve stability and peace in the region. They pledged to strive for a "real reconciliation for Israel, Arab states and Palestinians," for a curb on the "spiraling arms race," and to make a concerted effort to resolve the Arab-Israeli conflict.

The U.S. official said the statement would help bolster Arab members of the coalition once a "messy" ground war begins.

Elsewhere, Israeli officials said allied forces appeared to be having greater success in battling Scud missiles launched from western Iraq, although 10 to 15 mobile launchers remain intact. Pentagon officials echoed that optimistic assessment without providing any specific information about the Scud hunt.

But when asked by an Israeli television interviewer how long Israel could endure the missile barrages, Israeli Defense Minister Moshe Arens replied: "The situation . . . will not last for two months and even for one month. I simply estimate that a situation in which we continue to be neutral {while missiles continue to detonate} will not continue for a month."

German Chancellor Helmut Kohl called Bush on Monday to inform him of the new $5.5 billion German contribution to Operation Desert Storm, which was announced yesterday. The contribution from the Germans, added to money pledged last week by Saudi Arabia, Kuwait and Japan, brings total allied donations for the gulf effort to $41.5 billion for the first three months of 1991.

Staff writers R. Jeffrey Smith, Dan Balz, David S. Broder, Barton Gellman, Tom Kenworthy, George Lardner Jr., Thomas W. Lippman and Helen Dewar, correspondents Jackson Diehl and William Claiborne in Jerusalem, Michael Isikoff in Saudi Arabia, special correspondent Trevor Rowe at the United Nations and staff researcher Lucy Shackelford contributed to this report.