A mile-long oil sheen was spotted this morning during a Coast Guard flyover of ocean waters near the airport Reef Runway where the guided missile cruiser Port Royal was stuck for the past four days.

But Coast Guard officials said the 100-yard-wide sheen does not pose a marine pollution threat.

Pollution investigators aboard Coast Guard vessels in the area where the ship was stranded said the sheen is marine diesel, a very thin fuel which burns off quickly in sunlight.

There is no threat to the coastline or marine life from the sheen and a crew aboard the oil spill recovery vessel Clean Islands will remain on scene until the sheen burns away, the Coast Guard said.

"We are confident the small sheen will burn away this morning," said Cmdr. Kathleen Moore, the U.S. Coast Guard Alternate Captain of the Port and a senior officer responsible for oil spill response in the main Hawaiian Islands.

According to Coast Guard estimates, approximately seven to eight gallons of diesel fuel were spilled. It's not clear yet if the fuel is from the Port Royal or from one of the nine other vessels used to free the USS Port Royal, a billion-dollar guided missile cruiser home-ported at Pearl Harbor. The ship ran aground about 8:30 p.m. Thursday while transferring some of those who had been aboard the ship to a small boat.

Attempts to free the boat over the past several days were unsuccessful until this morning.

The damaged ship has been towed back to Pearl Harbor .