CLAYMONT, Del. (AP) _ An oil tanker from Uruguay carrying 19 million gallons of fuel ran aground early this morning in the Delaware River just south of here, spilling an undetermined amount of fuel, the Coast Guard said.

''At this point, we don't know what caused the ship to run aground and we don't know how much oil has spilled,'' said Petty Officer Tom Welch, with the Coast Guard's marine safety office in Philadelphia.

''We're just trying to concentrate on getting as much oil as we can pumped out of the ship and onto barges to prevent more oil from leaking out,'' he said.

Lt. Lewis Corcoran, also with the marine safety office, said authorities received a report shortly before 5 a.m. that the tanker Presidente Rivera, which was en route to Marcus Hook, Pa., had run aground about a mile off shore.

The vessel was carrying 19 million gallons, or more than 400,000 barrels, of No. 6 oil, which Corcoran described as a medium to heavy fuel.

''Right now we're just trying to contain the oil with booms,'' Welch said. ''We haven't been able to determine this morning how much oil has already spilled.''