CORONADO, Calif. (AP) _ About 500 gallons of oil spilled into San Diego Bay, fouling a mile-long section of beach and killing at least 28 birds, authorities said.

The Coast Guard was trying to determine where the spill originated, but the thick sludge that darkened part of the Silver Strand beach was identifed as diesel fuel, said Lt. Cmdr. Steve Mojonnier of the Coast Guard Marine Safety Office in San Diego.

The spill near the Navy's Sailor Association marina on the bay was discovered Wednesday morning. About 65 people, mostly Navy personnel, cleaned up more than half the slick with crafts that act like vacuum cleaners, said Lt. Cmdr. Rick Sullivan, the Navy clean-up coordinator.

The rest of the oil was dispersed rapidly and drifted out of the bay with the tide, he said.

Sailors pulled 28 dead, oil-soaked ducks from the area and 35 others were found alive. But Scott Drieschman, Sea World's curator of birds, said he ''the prognosis ... is dismal.''

''If we can save 50 percent of those that are alive right now, I would be very surprised,'' he said.

The spill occurred near the nesting area of the Least Terns, but the endangered species of bird don't arrive there until April, said California Game Warden Russ Gomez.