GALVESTON, TEX. -- The threat of a major oil spill in the Gulf of Mexico increased after two new explosions rocked a crippled oil tanker burning out of control yesterday, Coast Guard officials said.

Vessels equipped with firefighting and cleanup equipment were being rushed to the Norwegian supertanker Mega Borg in hopes of avoiding what could become a catastrophic spill.

The giant ship, anchored 57 miles southeast of this coastal city, was first hit by an explosion early yesterday after it had begun transferring its cargo of Angolan light crude oil to another tanker. Two crew members were killed and two are missing; 17 were injured.

Officials are concerned that the fire on board could rupture the vessel's cargo tanks and send some or all of the 38 million gallons of crude oil remaining on board gushing into the gulf.

While rescue vessels sprayed the ship to try to keep its tanks cool, two smaller explosions ripped through the vessel yesterday afternoon, shaking the Coast Guard's confidence about its ability to prevent a major spill.

Coast Guard Capt. Thomas Greene said the explosions did no visible damage to the ship's hull but that the flow of oil from the craft accelerated after the blasts.

"It seems to be coming in burps. Sometimes it's a very light sheen coming from the ship. Other times it's a larger discharge," he said.

Greene estimated that as much as 17,000 gallons of oil had leaked. A slick 11 miles long was floating in the gulf earlier, but Greene said natural evaporation and the movement of the sea had dissipated it.