BP cements oil spill plan

New Orleans - BP got the go ahead to seal in cement its runaway well in the Gulf of Mexico on Thursday, one of the final steps to plug the oil spill at the center of the worst US environmental disaster.

US officials cautioned that a great deal of clean-up work remained and that the long-term impact of the disaster could be felt for years, even decades. In the long-awaited breakthrough, BP brought the well under control on Wednesday after pumping heavy drilling fluid into the busted Macondo well for eight hours, forcing the oil back down into the reservoir miles beneath the seabed.

We "have reached a static condition in the well that allows us to have high confidence that there will be no oil leaking into the environment", spill response chief Thad Allen told reporters at a White House briefing.

BP said it would begin on Thursday to cement over the well, permanently shutting it in, after Allen authorised the British energy giant to proceed.

The US pointman also said, however, that he had "made it clear" to the company that the cementing should "in no way delay the completion of the relief well", expected to be finished in mid-August to seal the well permanently.

"So, the long battle to stop the leak and contain the oil is finally close to coming to an end. And we are very pleased with that," President Barack Obama said. "Our recovery efforts, though, will continue. We have to reverse the damage that's been done."

It took 106 days to shut the well down in the wake of a devastating explosion April 20 that killed 11 workers and sank the BP-leased Deepwater Horizon rig, unleashing a torrent of oil into the Gulf.

At 4.9m barrels - or enough oil to fill 311 Olympic-sized swimming pools - the disaster is the biggest maritime spill on record.

It threatened the fish and wildlife-rich US Gulf coast with environmental ruin and plunged residents of coastal communities into months of anguish over their livelihoods and the region's future.

A government report released Wednesday found that a third of the oil was captured or mitigated through burning, skimming, chemical dispersion and direct recovery from the wellhead.

Heat from the sun helped some of the chemicals in the crude evaporate. Waves and currents broke the slick up into smaller patches. Then the microbes which feed on natural oil seeps in the Gulf got to work, it said.

"At least 50% of the oil that was released is now completely gone from the system," said Jane Lubchenco, head of the national oceanic and atmospheric administration.

"And most of the remainder is degrading rapidly, or is being removed from the beaches."

But Lubchenco was quick to stress that scientists will not be able to determine for a long time the full extent of the damage.

"The oil that was released and has already impacted wildlife at the surface, young juvenile stages and eggs beneath the surface, will likely have very considerable impacts for years and possibly decades to come," she told reporters at the White House briefing.

The problem, she explained, is that oil is still toxic even when it has been broken down into very small droplets.

About 24% of the Gulf's federal waters remain closed to fishing, and even when fishermen are able to fill their nets they fear consumers might not believe the seafood is safe to eat.

With tourists likely to avoid Gulf beaches for years and oil industry jobs under threat from Obama's moratorium on new deep sea drilling permits, the future remains bleak for many coastal communities.

BP, meanwhile, is hoping to rebuild its shattered reputation but must also meet the claims of thousands  and businesses whose livelihoods have been washed away, while a mammoth civil trial looms.

BP senior vice president Kent Wells expressed relief that 20 days after the flow of oil in the sea was stemmed with a temporary cap "it's very difficult for us to find any oil anywhere on the surface."

He refused, however, to declare victory until the well is permanently sealed.