SEATTLE (Reuters) - As much as 1,500 gallons of used motor oil leaked from an above-ground storage tank in Washington state into a creek that flows into the Yakima River, vital to the apple-growing state's agricultural hub, officials said on Monday.

The cause of the spill on Sunday from the tank at a former feed lot near Sunnyside, about 170 miles southeast of Seattle, was under investigation.

Department of Ecology spokeswoman Joye Redfield-Wilder said the oil posed a threat to otters, waterfowl and fish as well as orchards and other crops in the area.

"In a couple of weeks, the canals will all be full and (farmers) will be watering their crops and their orchards, so we want to get this cleaned up," Redfield-Wilder said.

The Washington state Department of Ecology said its workers installed absorbent pads and protective booms at several sites, including about 900 feet upstream of the mouth of Sulphur Creek and at a fish hatchery on the Yakima River after Sunday's spill.

The slick could be seen in the water as far as 15 miles southeast of Sunnyside, the state said on Monday.

NRC Environmental Services, which the state hired to handle the cleanup, was using vacuum trucks to remove oil.

The Yakima River is an important water source for farm irrigation in south central Washington state. It is also a renowned trout fishing river.

(Editing by Peter Cooney)