The huge oil spill spreading across the Persian Gulf from a loading terminal off Kuwait is already much bigger than the 1989 Exxon Valdez spill in Alaska and could be one of the biggest ever, U.S. officials and scientists said yesterday.

The spill is a potential environmental catastrophe, experts said, that could foul much of Saudi Arabia's water supply and possibly complicate military operations in the war against Iraq.

With television already showing oil-soaked birds dying along the Persian Gulf coast, scientists said they feared a far worse environmental impact if Iraq mixes gasoline with the oil and sets it afire. Fire might burn off much of the crude oil on the surface of the water, but it could also emit vast clouds of thick smoke that would foul the atmosphere for hundreds of miles, environmentalists said.

The Saudi capital, Riyadh, and Saudi Arabia's Eastern Province, where more than 600,000 U.S. and allied troops have gathered for Operation Desert Storm, get almost all their drinking water from desalination plants on the gulf coast. The plants cannot process water contaminated by oil, experts said.

Satellite photographs taken by the National Oceanographic and Atmospheric Administration show oil flowing from a supertanker loading terminal, known as Sea Island, in the water east of the Mina al Ahmadi refinery complex. The terminal has apparently not been bombed in the air assault that began last week, and U.S. officials yesterday accused Iraq of simply opening the taps at Sea Island and on five tankers loaded with Iraqi crude oil that have been anchored off Kuwait since October.

Iraq blamed the United States for the spill in a complaint it took to the United Nations and said U.S. planes bombed two Iraqi tankers in the gulf on Tuesday. U.S. officials reported that the Navy damaged a tanker but said the relatively small slick that resulted was of refined petroleum rather than crude oil.

The flow of oil apparently began Wednesday. Energy Secretary James D. Watkins, who conferred by telephone yesterday with Saudi officials, said the impact will depend on "the quantity of oil, the direction of the current, the drift time to impact {on shore}, and the resources in place in the region to deal with it. The answer to all of those is, we think we can manage it."

He said Iraqi President Saddam Hussein had threatened to pour oil into the Persian Gulf and military planners had prepared for it, but he gave no details. The Energy Department's Sandia National Laboratory in New Mexico has conducted an extensive study of the potential environmental effects of the Persian Gulf War but the study is classified and the White House has refused to release it, congressional sources said.

The largest oil spill ever, about 184 million gallons or 4.3 million barrels, was caused by the blowout of Mexico's Ixtoc offshore well in the Gulf of Mexico in 1979.

The Sea Island terminal has the capacity to pump 100,000 barrels an hour, or 4.2 million gallons, a Pentagon official said.

Pentagon spokesman Pete Williams said "on the order of several million barrels" are in the water. That would make it far bigger already than the largest tanker spill in U.S. history, the Exxon Valdez, which dumped almost 11 million gallons of oil, or 260,000 barrels, into the waters of Prince William Sound. The Persian Gulf spill could keep growing because Sea Island is connected through onshore facilities to oil wells, not just to one tanker load.

The impact could be magnified because the Persian Gulf is a narrow body of water that will hold the oil in, rather than allowing to spread out and dissipate across a broad ocean, environmentalists said.

Until now, the western Persian Gulf has supported a vigorous and relatively healthy ecosystem, according to biologists who have studied in the region. As the scientists describe it, the western gulf is anything but an industrial wasteland, despite the extensive network of oil facilities on shore.

The spill has fouled beaches more than 20 miles south of Sea Island and is estimated to be about two miles wide.

"We're talking about the kind of environmental disaster the world has never seen before," said Mark Whiteis-Helm of Friends of the Earth. "This is just a drop in the bucket compared to what Saddam is capable of unleashing. He's got the potential to drop a Valdez-size spill every day for months."

"This oil spill, if it is not contained, will produce an ecological disaster," said Sayed Z. El-Sayed, a marine biologist at Texas A&M University.

In a telephone interview, El-Sayed and two researchers from Saudi Arabia described the western gulf as an area dotted with coral reefs and blanketed by fields of submerged sea grasses. There are mangrove forests and active commerical fisheries, "a fantastically rich shrimp fishery," El-Sayed said. Before the war, wealthy Saudis and Kuwaits fished for barracuda and sailfish.

"The coast supports very rich animal life," El-Sayed said. "Believe it or not, there are lots of dolphins and many sea turtles. There is much to be lost."

Desalination plants in the Saudi cities of Jubail and al-Khobar both appear to be in the path of the spill because Persian Gulf currents run counterclockwise. The plants operate by a process known as "reverse osmosis," in which filters screen out molecules of salt from the incoming sea water. If oil reaches these filters, they will become overwhelmed and useless.

"If oil gets into the sea water uptakes, it could destroy those plants," said Mahlon Kennicutt of the Geochemical and Environmental Research Group at Texas A&M. But some scientists said it is possible that the water intake for the plants could be relocated further below the surface of the water to avoid contamination.

Iraq has used oil as a weapon of war before. During the Iran-Iraq war, Iraqi forces struck with missiles and sabotaged pipelines, storage facilities, refineries, terminals, tankers, wells and offshore platforms.

"Saddam Hussein has shown himself capable of holding the environment as his hostage," said Richard Golob, publisher of Golob's Oil Pollution Bulletin, who has analyzed oil pollution in the gulf for the United Nations.

"Cleaning up such a large spill would be difficult, even in the best of times," said John Teal, an ecologist and oil spill expert at the Woods Hole Oceanographic Institute. "But this is a spill in the middle of a war zone. Nobody's going to be able to clean it up."

Oil spill specialists yesterday speculated that chemical dispersants could be used to break up the slick and help sink the oil. Booms and dredges could also be employed, but probably only to protect desalination plants.