DHAHRAN, SAUDI ARABIA, JAN. 26 -- The Saudi Arabian coast guard is mobilizing efforts to head off potentially crippling damage to major industrial and water-purification facilities posed by a Persian Gulf oil slick that now measures 30 miles long and 8 miles wide.

The floating mass of black crude, apparently unleashed when Iraq opened the valves on a Kuwaiti offshore oil-loading platform, has more than doubled its size since Friday, according to estimates provided by U.S. military officials today.

Depending on wind and water currents, experts say that in 10 to 15 days the slick could reach the coastal industrial city of Jubail, 100 miles south of the Kuwaiti border, where the Saudis operate a major water desalination plant that supplies up to 195,000 cubic yards of water daily to northeastern Saudi Arabia, including the capital of Riyadh. Other desalination plants threatened by the spill are located on the northern coast at Khafji and Khobar.

Jubail also is the site of the country's largest industrial center -- a sprawling complex that includes more than a dozen petrochemical plants, steel factories, oil refineries and other factories, all of which use sea water from the gulf as industrial coolants. They could be forced to close down if the waters become fouled by crude oil, said Niaz A. Khan, environmental specialist of the Royal Commission for Jubail, who is working with Saudi environmental officials to contain the damage.

The dramatic growth in the slick indicated that the oil is continuing to flow in large quantities into the gulf, intensifying fears of catastrophic damage to the marine environment, including thousands of dolphins, sea turtles, birds and other animals, according to environmentalists and oil experts here.

"It's quite terrible," said Khan. "This is the industrial backbone of the country. . . . If something happens to the waters, they would have to shut down all our industries" in Jubail.

"What you are seeing is a picture of a great disaster," said Abdulrahman Abdullah Awadi, the minister for cabinet affairs of the exiled Kuwaiti government and executive secretary for the Regional Organization for the Protection of Marine Environment. "The gulf is a closed sea. We think the {damage from the} oil is going to last quite a long time. . . . {Iraqi President Saddam Hussein} has a very strange system of thinking. I'm sure he's going to destroy anything that he can destroy."

A Saudi military spokesman also denounced Iraq's dumping of oil, calling it a "a deliberate act of environmental terrorism that will hurt the entire world." But some Saudi officials said today that, in addition to the environmental damage, they are equally alarmed about potential economic and health disasters resulting from the slick.

In addition to the oil slick, fires at oil production and storage facilities in Kuwait have polluted the air with a pall of thick, black smoke that is depositing an oily film upon coastal areas extending into Iran.

Earlier today, the danger from the oil slick receded slightly when gentle winds pushed it easterly, away from the Kuwaiti-Saudi coast and toward the center of the gulf in the direction of Iran, according to Saudi sources. Experts noted the slick was continuing to move south and, in light of circular currents of the gulf and normal wind patterns, it was highly likely that some of the oil would reach the Saudi coast.

"I can say with quite a bit of confidence that some of this oil will come down south. . . . It will come down to the eastern shores of Saudi Arabia," said Abdullah E. Dabbagh, director of the research institute of King Fahd University of Petroleum and Minerals, who has developed computer models to chart the course of oil spills.

Over the past few days, the Saudi coast guard has begun intensive monitoring of the slick, using helicopters to fly over the coastline near the Saudi border town of Khafji, where a British ITN television crew taped ramatic footage of waves of oil coming ashore and ducks drenched in heavy black crude.

In addition, Saudi officials in Jubail have begun deploying a protective system of rubber oil booms -- essentially floating barriers designed to block the oil from washing ashore. A spokesman for the Saudi petroleum ministry said the country has "long experience" dealing with oil spills and is well prepared to cope with the consequences of this one.

"We're working to block off all the facilities that use water," said the spokesman, who asked not to be identified. "We are really very confident that we will prevent it from causing any damage."

But other Saudi experts cast doubt on such comments, noting the reported dimensions of the slick. Although there have been conflicting numbers offered in recent days, the petroleum ministry today estimated that 6 million barrels, or about 250 million gallons, of oil have been dumped in the gulf so far -- far greater than the 11 million gallons spilled during the 1989 Exxon Valdez disaster in Alaska.

"This is of such a great magnitude that we never dreamed of having something like this," said Dabbagh.

Nevertheless, there has been some confusion about the amount of oil involved. Kuwaiti cabinet minister Awadi told reporters here today that the oil was being dumped from the terminal point of a pipeline extending from the Ahmadi oil-storage facility in Kuwait to Sea Island, a loading facility for large tankers about 10 miles offshore. Because the facility is located in Iraqi-controlled territory, it is impossible to gauge the flow of oil into the gulf, but the pipeline is believed to be capable of pumping 100,000 to 200,000 barrels of oil, or 4.2 million to 8.4 million gallons, per hour.

Scott also declined to say whether the United States was considering military action to seize the facility or otherwise seek to cut off the flow of oil.

Capt. Niall Irving, a spokesman for British forces here, said the multinational forces are "drawing on anybody with the necessary expertise to see how we're going to deal with it. We can't afford to get this wrong. . . . The scale of this thing is really huge."

He added, "This is big and is going to take a great deal of thought and care to make sure we get it right. We will have only one chance to get it right."

Scott also said in response to a question that the U.S. military is "absolutely sure" that the slick was not caused by any bombing raids of oil or other industrial facilities by the United States or its allies.