Updated

A tanker spilled 30,000 gallons of crude oil into the Delaware River (search) between Philadelphia and southern New Jersey, creating a 20-mile-long slick that killed dozens of birds and threatened other wildlife, federal officials said Saturday.

Private contractors were called in to skim oil from the surface of the water and place thousands of feet of boom to contain the floating slick.

U.S. Fish and Wildlife Service (search) officials said 50 birds were dead from the spill, 300 others were affected and fish also were threatened. A stretch of the busy river was closed to commercial and recreational traffic while the spill was being cleaned up.

"We're working very quickly and diligently to expedite the cleanup," said Coast Guard Petty Officer John Edwards.

Two tugboats were guiding the ship to a pier Friday night when a tugboat skipper noticed the spill, said Coast Guard Capt. Jonathan Sarubbi, officer in charge of the Port of Philadelphia. The ship listed 8 degrees to the left at about the same time, he said.

The crew notified the Coast Guard and began transferring oil from the leaking tank to another tank on board. The leak was stopped within an hour.

The cause of the spill was still under investigation, Sarubbi said.

The tanker, the Athos I (search), registered in the Mediterranean island nation of Cyprus, was carrying 325,000 barrels of oil from Venezuela, said Jim Lawrence, a spokesman for the vessel's owner, Greek shipping company Tsakos Shipping and Trading SA.

The nearly 750-foot-long ship, built in 1983, was last out of the water for maintenance in April, Lawrence said. He said it had never before spilled oil.

It was the worst spill on the Delaware River since 1995, when strong wind pushed a tanker away from a refinery dock in West Deptford, N.J., snapping a fuel line that spilled 40,000 gallons. In 1989, a tanker ran aground near Claymont, Del., spilling 300,000 gallons of heating oil into the river.

About one million barrels of oil come through the Port of Philadelphia each day.