The massive oil slick menacing Persian Gulf coasts has more than doubled in size in the past 24 hours, U.S. officials said yesterday, and President Bush announced he would send a team of experts from the United States to Saudi Arabia to assist in the emergency containment effort.

The Defense Department said Iraq has flown at least two dozen of its warplanes and cargo aircraft to Iran, as the allied air bombardment of Iraq continued and U.S. military commanders said their primary emphasis had shifted from strategic installations to battlefield-connected targets.

Iraq fed the lengthening slick by pumping more crude oil into the gulf from the Sea Island supertanker terminal about 10 miles off the Kuwaiti coast, U.S. officials said. The United States and allied nations groped for a way to stop the flow and contain the oil that reportedly was seen as far as 70 miles south of the source. Part of the oil near the terminal was burning, sending a huge cloud of smoke over the eastern gulf toward Iran, but officials reported that the fire had grown smaller during the day.

Pentagon spokesman Pete Williams called the slick "the worst environmental disaster in the history of the Persian Gulf region." Military officials said they were studying options, including aerial bombardment, to stop the outflow.

But officials insisted that the oil slick would have no effect on the Persian Gulf War and that efforts to contain it would not interrupt military operations. "Our job is to fight the war right now," one senior official said of the allied forces' priorities, adding that perhaps Iraqi President Saddam Hussein had hoped "that they'd all drop their guns and pick up sponges."

The release of the oil slick added to an ongoing sense that Saddam might have other unexpected cards to play. The overall extent of damage caused to Iraq and its military force after 10 days of continuous allied bombardment remained uncertain, as neither the Pentagon nor the U.S. Central Command in Riyadh, Saudi Arabia, released anticipated bomb damage assessments. U.S. officials said again yesterday that such information would be made public soon.

News that Iraqi planes had landed in Iran came after several previously unconfirmed reports that Iraq might be attempting to shelter its air force by sending it out of the range of allied bombers. Pentagon officials said they were trying to determine whether the pilots of the planes, about a dozen fighters and a dozen tranports, had defected or were seeking shelter as a way for Iraq to husband its resources for later.

Secretary of State James A. Baker III said Iran had assured the United States that it intends "to remain totally neutral" in the conflict and that any aircraft that land there "will remain in Iran for the duration of the conflict." Iran's ambassador to the United Nations, Kamal Kharrazi, said the planes would be "seized."

U.S. military officials said that allied warplanes were shifting their emphasis from the strategic communications and industrial sites they have targeted over the last 10 days to locations more directly concerned with battlefield support. With Iraq's nuclear capability destroyed and its chemical and biological weapons production facilities badly damaged, they said, allied pilots continued to pound Iraq's elite Republican Guard, transportation lines and bridges, as well as storage and ammunition dumps. These stepped-up attacks were seen as part of a strategy to counteract the surprising ability of the Iraqi army to repair damage to airfields, communications lines and other targets.

U.S. officials in Riyadh provided some anecdotal information about the effect of the bombing yesterday, including a report from pilots that a "tremendous fireball" could be seen 200 miles away after one attack on the Guard.

Allied fighter planes, engaging in what Marine Maj. Gen. Martin Brandtner, deputy director of operations for the Joint Chiefs of Staff, described in Washington as a "furball dog fight" over Iraq, shot down three Iraqi MiG-23s yesterday, bringing to 22 the number of Iraqi planes downed in combat. Another 23 planes have been destroyed on the ground. No allied planes were reported lost yesterday.More Scud Missiles Intercepted

The Pentagon said that Iraq launched four Scud missiles at Israel and one at Saudia Arabia yesterday, but U.S. Patriot missiles successfully intercepted them. No serious damage was reported.

As the fighting continued with a steady rhythm, the largest U.S. anti-war demonstration since the crisis began was held in Washington. A crowd of tens of thousands -- so dense that it formed a continuous parade 30-people wide for more than three hours -- provided a visual symbol of the opposition to the war in this country.

Overseas, nearly 200,000 protesters marched in Bonn, with another 30,000 demonstrating in Berlin. "We are not against the Americans," Evangelical Church Bishop Gottfried Forck told the peaceful crowd in Bonn. "We are against the recklessness of American policy."

As security officials continued to be on alert for possible terrorism connected to the gulf war, a car in southern Turkey that appeared to be wired with explosives blew up near the Incirlik air base that U.S. planes are using to launch strikes in northern Iraq. There were no injuries or damage from the blast. In Paris, a bomb exploded outside the main offices of the French newspaper Liberation. Police said it was the first terrorist attack there related to the war. There were no injuries.

Iraq opened a new radio link to the world yesterday in what appeared to be an effort to generate support among Arab populations in other countries. Called "Mother of Battles" radio, its signals appeared to be coming from Kuwait. Mixing music and poetry, the radio broadcast appealed to Arabs to join forces with Iraq. "Do you not feel proud to see us stand up against all the Arabs' enemies, not scared or frightened?" it asked. "Then why do you not join us?"

The British Broadcasting Corp. said it had been able to monitor the signal on two shortwave and three medium wave frequencies, suggesting attempts to reach a wide audience. "By God, tell me whether you are not pleased to have brothers who enjoy such strength and such determination," one message said. "We now represent the Arabs' awakening from the ocean to the gulf." Saddam described the war when it broke out as the "mother of battles."

Pentagon officials, responding to reports of civilian damage and casualties in Iraq as a result of the relentless aerial attacks, said allied planes have damaged no religious sites. They offered no information about civilian casualties, saying they had no way to assess that number.

But the controversy over civilian damage in Iraq continued yesterday with another broadcast from Cable News Network's Peter Arnett, who reported on his trip to Najaf, about 100 miles south of Baghdad, and said it was the "fourth day in a row I was shown damage to civilians and what Iraq officials described as nonmilitary targets." Arnett said he saw "huge bomb craters" in the streets, about "a dozen houses flattened," and several automobiles and a bus that "were just twisted wreckage at one locale." He also reported that he had been told of civilian deaths and injuries by some of the people.

Arnett also reported "pretty heavy" car and truck traffic on the drive from and to Baghdad. He said he saw a busy downtown market in one city and farmers at work in fields along the roads.

The Pentagon's Williams said that while allied pilots are under strict orders to avoid sensitive sites, Saddam bore the responsibility if he had chosen to put military facilities near religious or cultural shrines. "We're going after the military infrastructure of Iraq," he said. He added that the "very deliberate pace" of the operation reflected the desire "to try to minimize civilian casualties."

The civilian damage issue also drew warnings from the Soviet Union yesterday, when new Foreign Minister Alexander Bessmertnykh questioned "the scale" of the allied bombardment of Iraq, saying destruction of that country was not in accord with the U.N. resolutions. After meeting with Baker, he said what has happened to date is in "complete accord" with the resolutions but expressed concern that the conflict could widen dangerously. Iraqi Planes Escorted in Iran

Pentagon officials said they were continuing to seek out information about the Iraqi airplanes in Iran. Tehran radio, quoting Iran's Supreme National Security Council, said the latest seven Iraqi warplanes had penetrated Iranian airspace during the day and were escorted by Iranian jets to an airfield. The report said one plane exploded and two others were damaged.

Baghdad radio said it was seeking the return of the pilots and the planes, but the Iranian security council said later, "Out of concern for Iran's neutrality in the war, the council warns the belligerent parties that they should refrain from using the Iranian airspace in any form." It said it would confiscate any planes until the end of the war. U.S. officials said allied pilots had orders not to violate Iranian airspace.

Navy Rear Adm. Mike McConnell, director of intelligence for the Joint Chiefs, said the planes appeared to have landed "without any kind of restrictions on their ability to come into the country." He said U.S. officials did not know whether the flights were "prearranged or whether it was defection or whether it was an opportunity to put them in a safe haven." Officials said the planes were both civilian and military and the estimated two dozen or more in Iran had arrived there over a period of days. Some of the commercial aircraft flew to Iran about the time of the Jan. 15 United Nations deadline for Iraq's withdrawal from Kuwait.

As U.S. intelligence officials puzzled over the Iraqi planes in Iran, the oil slick occupied the attention of other government experts. U.S. officials reported that some of the oil, in the vicinity of the Sea Island Terminal, was burning, although they said the fire appeared to diminish as the day went on. Crude oil becomes harder to ignite the longer it is in the water. But the fire from the slick, as well as from fires at Kuwaiti oil facilities sabatoged earlier in the week, threw up a huge cloud of smoke that was blowing east in the 550-mile long by 150-mile wide gulf.

Officials in Saudi Arabia and the United States said they were most concerned about the threat to desalination and industrial facilities down the Saudi coast. The slick appeared to move away from the coast during the day, but given normal winds and currents in the gulf, experts there believed it would return to Saudi coastal areas later.

The team ordered sent to the gulf will be headed by the U.S. Coast Guard and will include representatives from the Army Corps of Engineers, the Navy Supervisor of Salvage, the Environmental Protection Agency and the National Oceanic and Atmospheric Administration. They will be under the supervision of Defense Department officials while there because they will be operating in a combat zone, Coast Guard spokesman Lt. Cmdr. James Simpson said. He said the group would not take specific equipment for combating the spill, but would help assess what steps should be taken to do so.

Jack O'Dell, a Coast Guard spokesman, said that private companies that specialize in fighting spills likely would be asked to lend equipment and expertise. "This would stress out any equipment that is kept on scene," O'Dell said. U.S. officials did not have precise information about the amount of oil already in the water, saying only that "millions of barrels" had been dumped.

Saudi officials have begun beefing up a protective system of rubber barriers designed to prevent the oil from coming ashore and to protect onshore facilities that use water. They predicted they could cope with the slick, but other experts warned that the size of the spill could overwhelm the Saudis' capabilities.

The Pentagon's Kelly said that while the black goo could clog up ships' machinery, the Navy had no intention of going through the slick as part of any military operation. Another government official said that while public statements emphasized the potential environmental damage of the spill, the Pentagon was "far less concerned with the environnmental effects than the tactical effects," adding that "their objective is to get it shut off and protect the desalination plants. Containment is well down the list."

Baker announced yesterday that Saudi Arabia had agreed to contribute $13.5 billion to help cover the cost of the war during the first three months of this year, which was identical to the Kuwaiti payment announced Friday. Together with $9 billion newly pledged by Japan, the United States has raised $36 billion to help defray the cost of the war to American taxpayers. Staff writers Thomas W. Lippman, Don Oberdorfer, Don Phillips, Dana Priest and R. Jeffrey Smith in Washington, Marc Fisher in Germany and Michael Isikoff in Saudi Arabia contributed to this report.