The United States said Thursday it has ended all litigation stemming from the Exxon Valdez oil spill, closing the books on a 25-year legal saga over one of the 20th century's worst environmental disasters.

The Justice Department cited the recovery of duck and sea otter populations as the reason for its decision not to press for more damages.

Around 11 million gallons (42 million liters) of crude spilled off the coast of Alaska in March 1989 when the tanker ran aground on a reef. Images of birds covered in oil traumatized America.

In its immediate impact, the spill is estimated to have killed up to 250,000 sea birds, 2,800 sea otters, 300 seals and 22 killer whales.

The longer term effects turned out to be worse than expected. The herring industry, for instance, which until then was bustling, collapsed in 1993.

A legal battle spanning the 1990s and 2000s ensued as Exxon was ordered to pay damages and the company filed appeal after appeal to try to get the amount reduced.

The oil giant ultimately had to pay hundreds of millions of dollars in damages and interest and spent billions cleaning up the mess.

A ruling in 1991 allowed Alaska and the federal government to demand another 100 million dollars from Exxon in the event animal and plant populations declined significantly.

In 2006, it was estimated the populations of harlequin ducks and sea otters in the Prince William Sound, which was ground zero of the disaster, continued to suffer from the presence of subsurface oil residue. US authorities called on Exxon to do something about it.

But nine years later "scientists have concluded that exposure to the subsurface oil is no longer biologically significant to these species," the Justice Department said in a statement Thursday.

Populations of harlequin ducks and sea otters have returned to their pre-spill levels, so the government plans no further litigation against Exxon, the department said.

US: Alaska wildlife recovered since 1989 Exxon disaster
Washington (AFP) Oct 15, 2015 - Key wildlife populations like otters that were devastated in the 1989 Exxon Valdez oil spill in Alaska have fully recovered, the US Justice Department said Thursday.

With especially sea otters and harlequin duck now recovered "to pre-spill population levels," the department said it had no grounds to exact more damages from the oil giant, now called ExxonMobil.

"Scientists have concluded that exposure to the subsurface oil is no longer biologically significant to these species," the department said in a statement.

"Accordingly, the governments have decided to withdraw their 2006 request to Exxon to fund bio-restoration of subsurface lingering oil patches," it said, referring to the federal and Alaska state authorities.

Exxon's huge tanker ran aground in Prince William Sound in March 1989, spilling nearly 11 million gallons (42 million liters) of crude oil from Alaska's North Slope fields into the waters, affecting 1,500 miles (2,400 kilometers) of coastline that included several national parks and wildlife refuges.

At the time it was the world's largest oil spill, eclipsed in 2010 by BP's Deepwater Horizon spill in the Gulf of Mexico.

Exxon was forced to pay billions of dollars in fines and cleanup costs. But in 2006 the Justice Department reserved the right to further fine the company if the habitat for the two species was not fully restored and their populations still injured by the spill's effects.

"Our action today allows us to celebrate all that has been accomplished in Prince William Sound since the spill," said John Cruden, Justice Department assistant attorney general.

Officials said their still remains significant signs of the oil under the soil and in rocks in the area.

"Although the lingering oil is largely in subsurface soil or rocks, it does have the potential, if disturbed, to expose intertidal resources to oil, and its presence can be disturbing to people who come across it," said Lois Schiffer of the National Oceanic and Atmospheric Administration.

"The real question is whether it is better to intervene or to leave it to break down over time."