Exxon Mobil Corp. was ordered June 15 to pay about $500 million in interest on punitive damages for the Exxon Valdez oil spill off Alaska, nearly doubling the payout to Alaska natives, fishermen, business owners and others harmed by the 1989 disaster.

The Exxon Valdez leaks crude oil into Alaska's Prince William Sound on April 9, 1989. AP PHOTO

The ruling was issued by the 9th U.S. Circuit Court of Appeals in San Francisco. The decision would double the average payout of about $15,000 for the nearly 33,000 claimants.

In June 2008, the U.S. Supreme Court set punitive damages at $507.5 million. But two months later, the high court declined to decide whether Exxon Mobil must pay interest on the punitive damages awarded in the United States' worst oil spill and sent it back to the appeals court.

"We're just happy that we've cleared another hurdle and hopefully we can get the case tied up as soon as possible," said Stanford University law professor Jeffrey Fisher, an attorney for the plaintiffs.

Exxon could appeal the decision on the interest payments to the Supreme Court.

The case grew out of the 1989 running aground of the Exxon Valdez, a supertanker that dumped 42 million liters of crude oil into Alaska's Prince William Sound, fouling nearly 2,000 km of coastline.

Plaintiffs originally were awarded $5 billion but that amount was cut in subsequent appeals by Irving, Texas-based Exxon.

A jury decided in 1994 that Exxon should pay $5 billion in punitive damages. In 2006, a federal appeals court cut that verdict in half.

The Supreme Court last June slashed the $2.5 billion punitive damages award to $507.5 million.