Irving, Texas-based Exxon Mobil, the world's largest publicly traded oil company, has already paid US$900 million in a 1991 civil settlement.

But a "reopener" provision in the deal allowed the government to seek up to US$100 million extra for unforeseen damages. Exxon reported a US$36 billion profit last year.

The government sent the new cleanup plan to Exxon on Thursday. If Exxon refuses to pay, the government faces a Sept. 1 deadline to file an official claim in court.

Exxon will study the government's request but "nothing we have seen so far ... indicates that this request for further funding from Exxon is justified," company spokesman Mark Boudreax said in a statement.

Exxon agrees that there are "small pockets" of lingering oil, but those are limited to less than two-tenths of 1 percent of the Prince William Sound shoreline, Boudreax said.

"There is no scientific evidence that this oil ... could cause damage to any population or species," he said.

Trustees overseeing Exxon's previous US$900 million payment still have about US$145 million on hand and that money should be used for additional cleanup, he said.

But government studies done since 2001 have found that there is still oil residue left just below the surface of Alaska's beaches from the spill.

"After extensive review it is clear that populations and habitat within the oil spill area have suffered substantial and unanticipated injuries that are attributable to the Exxon Valdez oil spill," said Alaska Attorney General David Marquez.

Marquez said he was disappointed by Exxon's initial comments. "I hope they will take a long and serious look at our proposal," he said.

Crude oil from the grounded Exxon tanker spread to 1,087 miles (1,750 kilometres) of coastline, including the Chugach National Forest, three national parks, four national wildlife refuges and five state parks.

Oil from the spill killed about 250,000 marine birds, 2,800 sea otters, and wreaked havoc on shellfish, mussels and killer whales, according to government estimates.