The fiery collison of two supertankers off the coast of Tobago last night has poured huge volumes of oil into the Caribbean Sea in what appears to be the worst oil spill in history.

The collision of the two giant vessels, each longer than three football fields, has created a 25-square-mile oil slick in the azure Caribbean waters just a few miles off Tobago's tourist beaches.

At least 15 crewman were missing late tonight, a day after the collison at dusk Thursday, but 63 others had been rescued, several with serious burns.

U. S. Coast Guard officials estimated that by late today about 1.6 million barrels of oil had spilled from the two vessels, which reportedly were fully laden with 3.5 million barrels of crude oil and refinery products.

The total spill apparently has already exceeded the worst previous oil disaster, a blowout June 3 at an offshore well in the Gulf of Mexico, near the Yucatan Pennisula. That blowout, which has not been stopped, has leaked about 1.4 million barrels.

In the worst previous tanker accident, 1.3 million, barrels spilled from the Amoco Cadiz, which ran aground off the coast of France in March 1978.

The two vessels that collided last night were the 1,139-foot Greek-registered Atlantic Empress, which carried 270,000 tons -- 2 million barrels -- of oil, and the 1,066-foot Liberian-registered Aegean Captain, which carried 200,000 tons.

Together their cargoes amounted to about one-fifth of the daily petroleum consumption of the United States.

All but one of 35 crew members of the Aegean Captain were rescued, and many were taken to hospitals in the twin-island republic of Trinidad and Tobago for treatment of burns.

Only 14 of the 43 crew members of the Atalntic Empress have been rescued. Air and sea searches continued for the missing.

There was no immediate explanation for the collision, which happened about 20 miles east of the coast of Tobago. The U. S. National Weather Service in Miami said there was "nothing particularly unusual" about the weather around the island at the time.

By late today, the burning hulks of the two vessels had drifted to within seven miles of the coast and could be seen from the silvery white beaches that provide most of Tobago's earnings.

Residents of Roxborough, a fishing village on Tobago's east coast, could see thick columns of black smoke billowing thousands of feet into the sky.

Officials said the Aegean Captain, apparently the more damaged of the two, was listing badly and was in danger of sinking.

Liberia's admiralty counsel in the United States, Frank Wiswall, said Liberian investigators had been told the Atlantic Empress was carrying a full load of crude oil. Shipping sources said it was carrying Middle East oil to the United States.

Wiswall said the Aegean Captain had left a refinery at Curacao, a Dutch island 500 miles to the west, with a full load of refined oil products.

The Atlantic Empress, built in Denmark in 1974, is listed by the American Bureau of Shipping Record as owned by Branco Shipping Co., of Liberia but Liberian sources in the United States said its registry had recently been transferred to Greece.

The Aegan Captain, built by Hitachi Shipbuilding in Japan in 1968, is listed as owned by the Quadrant Shipping Co. of Liberia. CAPTION: Map, no caption, By Richard Furno - The Washington Post