Iraq set fire to oil installations in Kuwait yesterday, U.S. military officials said, but uncertainties about which facilities, and how much oil they contained, made it difficult to assess the extent of damage or the environmental impact.

There was no indication that the fires presaged the wider attack on the oil installations of the entire Persian Gulf region that Iraqi President Saddam Hussein has threatened. Saudi Arabia's vital oil facilities, pipelines and shipping terminals have not been affected.

U.S. military spokesmen in Washington and the Saudi capital of Riyadh said aerial photographs showed fires at oil wells and storage tanks in the Wafra field, in the neutral zone between Kuwait and Saudi Arabia. Unconfirmed reports from the region said Kuwait's major oil and gasoline storage tanks at Shuaiba and Mina Abdullah, closer to Kuwait City, also were burning.

If both areas are involved, "You're talking about almost the entire country's storage capacity," said Thomas Gochenour, a director of the Washington-based Petroleum Finance Co. and a former official with the state-owned Kuwait Petroleum Co.

"The question is whether it's full," he said, referring to Kuwaiti system of oil facilities. "I've heard completely conflicting reports: It's full to the brim or it was emptied by the Iraqis."

Saddam has said his forces will mine and ignite wells and refineries across Kuwait. Gochenour said such a conflagration could not be touched off until after Iraqi troops withdraw from Kuwait because it would cut off their retreat.

But the threat has sparked a scramble by scientists to assess the environmental effects of sustained fires and millions of tons of black soot lofted into the air.

"If Saddam carries out his threat, it will be a massive pollution event. It will be an environmental disaster," said Richard Small, director for thermal sciences at Pacific Sierra Research Corp in Los Angeles and an author of a study assessing environmental effects of war in the Gulf.

In the worst case, Kuwait and downwind regions could be blanketed by thick, roiling clouds of soot and ash, which would cause environmental damage and could limit the use of some of the U.S.-led forces' so-called smart bombs, which are guided to their targets by laser beams and television cameras. It is also possible that Iraqi troops could ignite oil-fed fires to foil ground attacks by U.S.-led forces.

Small estimates that some 450,000 tons of soot could be dispersed into the air if Iraqi forces ignited all 360 wells that were in operation when Kuwait was invaded.

"That's a massive amount of soot," Small said. If the westerly winds hold, soot would fall out over Kuwait, the Persian Gulf, southern Iran, and perhaps as far as Pakistan and northern India.

Since refineries and loading operations are on the coast, the raging fires and accompanying spills could also wreak havoc on the marine environment, possibly forcing the closure of water-treatment plants.

"In a worst-case scenario, a black tide would contaminate near-shore areas and cause serious damage," said Richard Golob, publisher of Golob's Oil Pollution Bulletin.

Some scientists, such as Carl Sagan of Cornell University, have even predicted that the soot clouds could alter weather around the world by shielding the Earth from incoming sun.

"The basic idea goes like this," Sagan said. "Let's assume we take Iraq at their word and they have mined every well in Kuwait, that they all go up and burn simultaneously."

While oil-fire fighters such as the legendary Red Adair say subsurface oil won't burn because of lack of oxygen and there isn't a wellhead fire they can't put out in a couple of weeks, Sagan and others contend that during a war, the fires will probably burn out of control, perhaps for months.

They fear that the dark sooty clouds would blanket the region and could cause crop failures and might even alter moonsoon rains. Sagan said yesterday that huge fires in Kuwait could lower average temperatures over North America by several degrees. Sagan earlier developed the theory that a superpower conflict could lead to a global "nuclear winter."

Other researchers discount such climate effects. "The fires won't alter worldwide climate," Small said. "There's just not enough smoke and the smoke and soot won't get high enough to alter temperatures significantly."

"It all sounds more like a severe air pollution episode than a climatic catastrophy," said Michael MacCracken of the Lawrence Livermore National Laboratory in California, who ran computer simulations on the effects of burning all of Kuwait's oil wells and refineries, a fire that would feed on more than three million barrels of oil a day.

Such a huge conflagration would cover Kuwait with some 25 micrograms of soot per cubic meter of air. As a comparision, MacCracken said that extreme pollution episodes involve 50 micrograms for the same volume of air, while the atmosphere around Los Angeles airport can contain 7 micrograms.

"Environmental catastrophe? I don't think so," said Bob Hall, an expert in oil production at the American Petroleum Institute in Dallas. Hall said that burning wells, storage tanks and refineries in Kuwait could produce serious pollution, but he believes the effects would be local, and would not result in mass starvation and freezing temperatures.

"Burning hydrocarbons is a way of life," Hall said. "All over the world, we're burning hydrocarbons every single day in amounts far exceeding what would be burned in Kuwait."

Heavy smoke could hamper the accuracy, and perhaps the use, of smart bombs, according to weapons experts.

"To varying degrees, the laser-guided and TV-guided missiles would be affected," said Steve Kosiak, an expert in aircraft weaponry at the Center for Defense Information here. "It all depends on the smoke and smoke coverage."

"The problem is that laser beams don't go through heavy smoke and stuff like that," said Anthony Siegman, a laser researcher and a professor of electrical engineering at Stanford University. "I doubt they'll be able to go through clouds of heavy smoke, but they may go through haze."

James P. Coyne, a retired Air Force colonel and fighter pilot, said the thick clouds would hamper bombing that depends on visual sighting of targets.

It was not clear yesterday what Iraq's motivation might have been for setting fire to the Wafra installations -- the only ones in Kuwait that, before the Aug. 2 invasion, were operated by an American company, Texaco Inc.

Asked at a briefing for reporters in the Saudi capital whether the fires might have been started by allied bombing, Lt. Col. Greg Pepin said they had been set by the Iraqis. "I can't get into the Iraqi mind, but yes, we do have evidence that they started them," he said.

Because no oil has been exported from Kuwait since the United Nations embargo imposed shortly after the invasion, fires in the Kuwaiti fields and refineries will have no impact on world oil supplies, which are ample.