President Bush acknowledged yesterday that he is "skeptical" air power alone can win the Persian Gulf War and announced he will send his two top military advisers to Saudi Arabia to gauge allied progress in the three-week-old conflict.

Defense Secretary Richard B. Cheney and Gen. Colin L. Powell, chairman of the Joint Chiefs of Staff, plan to leave later this week "to take a final look . . . {at} when the ground attack should begin," a senior White House official said.

Appearing tough and uncompromising at a news conference, Bush deflected questions regarding the timing of the ground war, while echoing the widespread Pentagon conviction that bombing sorties alone are unlikely to force Iraqi President Saddam Hussein to vacate Kuwait. The president also made clear he would not consider a cease-fire without "a credible, visible, totally convincing withdrawal."

Saddam must "say, 'I'm going to get out of Kuwait now, and I'm going to get out fast, and I'm going to do it so that everybody knows I'm not making this up,' " Bush said, apparently irked at recent speculation that the administration would accept anything less than an unconditional Iraqi retreat.

The president also indirectly disputed an assertion by Soviet Deputy Foreign Minister Alexander Belonogov that the war is "getting out of control" and doing "irreparable harm" to the Iraqi people. Although Bush made no reference to Belonogov, who spoke as he was leaving Moscow for talks with Iranian leaders, the president said the United States has gone to "extraordinary lengths" to spare civilians and non-military targets. "We are not trying to systematically destroy . . . Iraq," he added.

Pressed on the issue of whether Saddam could turn sympathy for the increasing Iraqi casualty toll into a wave of anti-American sentiment, Bush said, "I wouldn't be surprised if that's what he's trying to do" but added that victory by the coalition would create "renewed credibility for the United States." Bush also said: "What concerns me are the lives of our troops. . . . Saddam Hussein should be concerned about the Iraqi forces."

In the air war yesterday, allied aircraft reportedly bombed Baghdad and Saddam's home town of Tikrit as Iraqi radio announced an end to civilian gasoline, heating oil and other fuel sales because of critical shortages.

The allies unleashed 26 additional fighter and B-52 strike "packages" against Republican Guard units as part of the 2,800 sorties flown yesterday. For the second consecutive night Marine Harrier jets attacked a sizable pool of vehicles in southern Kuwait, hitting a convoy of 25 trucks that resulted in "multiple secondaries to include a fuel truck explosion," Maj. Gen. Robert B. Johnston told reporters in Riyadh, Saudi Arabia, yesterday.

Also for the second night in a row, the battleship USS Missouri wheeled into action, lobbing six rounds from its 16-inch guns on a long-range artillery battery that had been peppering allied ground forces. The shells destroyed one Iraqi gun, damaged another and shredded four others with shrapnel, Johnston said. Later, the Missouri unleashed 28 rounds against a radar control complex, "completely destroying" it, Johnston said.Syrian Forces Are Attacked

Syrian forces entered combat for the first time yesterday, Saudi Col. Ahmed Robayan announced, firing artillery at a reconnaissance force of 30 Iraqis attempting to cross into Saudi Arabia. After firing rocket-propelled grenades and small arms, the Iraqis retreated, the colonel added. But a news pool report from the front quoted U.S. Marine officers as saying Iraq struck twice, overrunning a Syrian position before being thrown back in the second attack.

After several days in which the Iraqi air force had remained virtually immobile, 10 more aircraft successfully dashed eastward to Iran yesterday, bringing to about 110 the number seeking sanctuary in the land of Iraq's former archenemy. Most of the planes that decamped yesterday were top-line fighters, joined by at least two transport aircraft flying from an air base near Baghdad, according to British and U.S. military officials.

Allied spokesmen again expressed confidence in Iran's pledge to impound all Iraqi aircraft until after the war. British Group Capt. Niall Irving said those preparing to flee have tended briefly to switch on radars to search for allied patrols. "If patrols are present, they switch off the radars and wait. Once the area is clear. . . the Iraqi aircraft make a dash for Iran."

Frustrated U.S. officers have noted that the combined size of Iraq and Kuwait exceeds that of California, complicating allied air patrols along the Iran-Iraq border.

As part of a plan to augment the B-52 fleet used to pound Republican Guard units, the Air Force yesterday moved some of the long-range bombers to a base in Fairford, England, a picturesque Cotswold village about 70 miles west of London. Other bombers already are flying from Saudi Arabia, Spain and the Indian Ocean island of Diego Garcia.

French aircraft reconnaissance photographs shown on ABC News last night suggested both the problem -- and opportunity -- facing the allied bombers. Tanks, reportedly belonging to the Republican Guard, were clearly visible but surrounded by 10-foot earthen berms that deflect any blast except that of a direct or near-direct hit.

British warplanes struck a warehouse and railway junction in southeast Iraq yesterday while six bombs scored "direct hits" on a railroad bridge, according to Capt. Irving.

In a daylight raid, Tornado bombers dropped 30 1,000-pound bombs on a complex of ammunition and Scud storage facilities in southern Iraq, scoring "a highly satisfactory result," Irving added. During night raids, Tornadoes dropped a total of 106 1,000-pound bombs on two ammunition dumps, one south of Baghdad, the other in southeastern Iraq.

Iraqi Foreign Minister Tariq Aziz, in a letter to Iraqi newspapers, said allied raids from Jan. 26 to Feb. 3 had killed 108 civilians and injured 249. The new figures indicated that a total of 428 civilians have died and more than 650 have been wounded since the war began Jan. 17, according to official Iraqi figures.

In his letter, Aziz said 37 civilian areas had been struck, including television and radio stations, houses, vehicles and an irrigation dam 170 miles west of Baghdad.

Residents living on the outskirts of the Iraqi capital reported hearing two window-shattering explosions shortly after midnight (4 p.m. Monday EST), and the Associated Press cited witnesses who saw soaring columns of fire and smoke. Travelers from Tikrit, the Iraqi president's home town, said the city came under intensive fire Monday and early yesterday. U.S. officials in Riyadh said there are several military installations near Tikrit, but denied attacking the town itself.

Visitors from Basra, Iraq's main port and the gateway to Kuwait, also reported numerous fires there, while refugees escaping from Kuwait to Jordan said raids on oil installations there had sparked raging refinery fires, churned up large clouds of soot and turned rainfall black.

"We couldn't even breathe," one oil company worker told the Reuter news agency in Ruweished, Jordan. "We could see the fire, and there were clouds of smoke. It rained the same day, and the rain was black with oil." The worker's wife said running water, electricity and telephones remain in service in Kuwait, but the country is plagued with shortages of rice, flour and soap.Harsh Words for Coalition

Pentagon officials interpreted the curtailment of civilian fuel sales, announced in a Baghdad radio broadcast, as further evidence that the bombing campaign against what pilots call POL targets -- petroleum, oil and lubricants -- has crippled Iraq's ability to refine its crude oil. With overnight temperatures in Baghdad sinking into the 30s, the cessation of heating oil sales is expected to aggravate living conditions in a city already stripped of power and most of its water system.

While the Iraqi army, navy and air force appeared to remain passive yesterday, a spokesman for Iraq's military continued to fling harsh words at the allied attackers. "The war criminal Bush and his allies, in addition to war merchants who were afraid of Iraq's might and strength and Iraq's future, start air raiding and shelling residential areas in a savage way that history did not know before," the spokesman declared in a radio broadcast.

U.N. Secretary-General Javier Perez de Cuellar, clarifying his complaints on Monday regarding allied planes allegedly bombing Jordanian truck drivers in Iraq, said yesterday that anyone hauling oil from Iraq would be in violation of international sanctions, but bombing is "a little too tough" a punishment.

In Riyadh, Johnston said "we are not specifically targeting the Jordanian civilian tankers," but cautioned that driving on main supply routes in Iraq "is a very risky proposition."

Saudi and U.S. officials said six suspects were arrested yesterday in connection with a shooting incident Sunday night in the port city of Jiddah, where two Americans were slightly injured by flying glass. The officials confirmed that the suspects were neither Saudi nor Iraqi, but released few other details. Offering a 1 million riyal ($267,000) reward for information related to subversive activities, the Saudi interior minister also warned subversives in a statement that they could face execution, banishment or amputation.

Baghdad Radio on Monday night began carrying what it called "special messages," which appeared to be coded calls for listeners outside Iraq to take unspecified action. A State Department official said the department had "no way of knowing what the messages meant," although he was aware that officials had seen various commentators speculating that the cryptic messages were calls to terrorists.

The radio broadcast, monitored outside the country, began: "Call, call from Hammad to Qutaybah: Implement all that is on the table and outside it." The announcer later said: "From the headquarters to 'Urwah: Implement the 'last meeting program.' " Another message said: "Call, call from Mahyub to Ayman, from Mahyub to Ayman: We are waiting to hear your voice and that of all the brothers. Do not hesitate. May god be with you."

Bombs exploded outside a Beirut bank in Lebanon and in a Citibank branch in Greece yesterday, while a U.S. Embassy attache's car was torched by a gasoline bomb in Amman, Jordan, officials said. In Peru, police said leftists killed three security guards linked to the U.S. Embassy in Lima.

U.S. officials reported that no additional Iraqi Scud missiles were fired at either Saudi Arabia or Israel. But White House officials confirmed reports in Jerusalem yesterday that Prime Minister Yitzhak Shamir has warned Bush in a letter of Israel's intent to retaliate immediately if Iraq uses chemical weapons or otherwise causes substantial Israeli casualties.

Shamir's letter, delivered to the White House Monday, is a "formal restatement" of the Israeli policy outlined to Deputy Secretary State Lawrence S. Eagleburger during recent visits there, an administration official said. According to the Israeli newspaper Maariv, Shamir told Bush, "I am sure you understand that if any attack on us causes a significant loss of life or if a non-conventional warhead is used, such a situation would become intolerable and would call for an immediate response by us."Timing of Ground Offensive

U.S. policy is "to urge Israeli restraint, but if they feel that they have reached a point where they must respond, we recognize their right to do so," the U.S. official added.

White House officials acknowledged that the upcoming trip to Saudi Arabia by Cheney and Powell -- the fourth for each since Iraq invaded Kuwait last August -- is a critical step in determining the proper timing for an allied ground offensive.

At least two of the secretary's trips have foreshadowed critical decision points in the crisis. As a consequence of Cheney's first trip, in early August, Bush announced the military deployment; during his last trip, in December, the final touches were put on what became the plan for Operation Desert Storm.

In his news conference yesterday, Bush said he would depend on the advice of his senior military officials, dismissing suggestions from congressional Republicans and others that the air war perhaps be extended into the summer. "I wouldn't go against sound military . . . doctrine in order to just delay for the sake of delay, hoping that it would save lives," the president said.

Repeatedly advised not to personalize the war or demonize Saddam, Bush yesterday allowed his personal antipathy toward the Iraqi leader to show through. Although removing Saddam from power is not a requirement for an allied victory, the president said, "Would I weep? Would I mourn if somehow Saddam did not remain as head of his country? . . . There would be no sorrow if he is not there."

Conditions for ending the war must include the withdrawal of Iraqi forces from Kuwait, immediate restoration of the Kuwaiti government and the installation of a supervisory force to keep the peace, Bush added. The allies would not accept an "I'll get out if you'll get out" compromise with Saddam, the president said. "We passed that. . . . Now we're in a war with this man, and he will comply . . . and then we can determine what niceties or what little details need to be done."

Bush, who will go to great lengths to avoid any analysis of his mood, insisted yesterday that his morale is high, that he is sleeping well and that he is not excessively troubled by the weight of decisions affecting the fates of thousands of soldiers, sailors and airmen. "I'm just so confident of how this thing's going to work out," he said, while acknowledging, "I can't tell you I don't shed a tear" for those lost in combat and their families.

The president described a letter he received from one war widow and a meeting last week with the families of others missing or killed in action in the gulf. "I came back with a sense of wonder at the way these spouses" are holding up, the president said. "We've had very few losses and yet, I got to tell you, I feel each one, but we're going to continue this and we're going to prevail," the president said at the conclusion of the news conference.

Staff writers Barton Gellman, John M. Goshko, Al Kamen and staff researcher Lucy Shackelford contributed to this report.