The tanker Exxon Valdez, maneuvering through ice-clogged waters with 1.3 million barrels of oil, ran aground on a charted reef Friday and spilled 200,000 barrels, closing the nearby port of Valdez and creating an oil slick five miles long, the Coast Guard said.

The oil terminal is a major American oil port and source of one-fourth of domestic crude that travels down a pipeline from Prudhoe Bay.The 987-foot tanker, bound for Long Beach, Calif., went aground on the charted reef 22 miles south of the port of Valdez, traveling through an area where it should not have been, said Chief Mark Peterson of the Coast Guard Marine Safety Office in Valdez.

He said the Exxon ship was trying to avoid large chunks of ice known as growlers. The icebergs come from nearby Columbia Glacier and often can be found in the area at this time of year.

The Coast Guard, the state Department of Environmental Conservation and crews from Exxon and the Alyeska Pipeline oil terminal mobilized to respond. Gov. Steve Cowper flew to the scene. Officials were assessing environmental damage and harm to fishing grounds.

Valdez harbor was closed, said Coast Guard spokesman Ed Moreth.

Exxon acknowledged the ship took a detour into treacherous waters off established tanker routes, and Exxon Alaska coordinator Don Cornett in Anchorage said, "Look at the chart and you can see the reef," but he declined to establish blame for the accident.

Oil leaked rapidly for 12 hours, and a slick stretched five miles south of the tanker toward the center of Prince William Sound - an important fishing area - before the leak slowed to a trickle by afternoon, said Petty Officer John Gonzales.