Washington, May 28 (DPA) US President Barack Obama extended a moratorium on new deepwater offshore drilling Thursday as oil giant BP's critical top-kill operation had likely temporarily stopped an oil leak that has already created the worst oil spill in US history.

As much as 540,000 barrels of oil may have already spewed into the Gulf of Mexico since the Deepwater Horizon rig exploded April 20, at a daily rate five times more than originally thought, according to new government estimates. The new leakage figures surpass the total oil spilled during the infamous 1989 Exxon Valdez spill in Alaska.

US Coast Guard Admiral Thad Allen said BP had managed to stop the leak, in an interview with the Los Angeles Times. Engineers had forced enough heavy mud into the damaged well head to reduce the upward pressure and prevent oil and gas from gushing out.

A Coast Guard spokeswoman later clarified that Allen "did not declare success". The top-kill procedure would temporarily stop the leak "simply because of the pressure from the mud injection". That did not necessarily mean the operation had worked, though "things were going to according to plan", spokeswoman Lisa Novak said.

The top-kill operation, launched at 1 pm (1800 GMT) Wednesday, marks the first time that BP has tried to permanently seal off the well. Past efforts have focussed on siphoning off the spewing oil.

"This procedure offers no guarantee of success," Obama said in an afternoon press conference. Obama is to tour the oil-slicked coast Friday for a second time to assess efforts.

BP has also sounded a more cautious note. Managing Director Bob Dudley told broadcaster CNN Thursday morning that the "operation is proceeding like we expected". A full update was due later in the afternoon.

Obama said he was extending a moratorium on new permits for deep water offshore oil drilling for six months, and halting permits or explorations of drilling opportunities off coastlines in Alaska and Virginia. Sharply tightened regulations were also planned, focusing on stricter safety standards and controls on oil rigs.

Obama heavily criticised the "cozy" and "scandalous" relationship between US regulators and the oil companies, just as the head of the Minerals Management Service (MMS), which is charged with watching over the oil industry, tendered her resignation.

Elizabeth Birnbaum, director of the MMS, submitted her resignation "on her own terms and her own volition", according to a statement from Interior Secretary Ken Salazar. Obama said it had been entirely Salazar's decision.

Also Thursday, the US government said the amount of oil that has been flowing into the Gulf is much more than originally estimated.

It is now believed that anywhere from 12,000 to 25,000 barrels of oil per day were spewing out of the well, up dramatically from the initial prediction of just 5,000 barrels per day.

Marcia McNutt, director of the US Geological Survey, said the estimates were still preliminary. Predicting the flow rate was still a "highly dynamic, complex situation".

The Obama administration and BP has long faced criticism for underestimating the pace of the leak. Obama acknowledged that "our efforts fell short" in making an accurate estimate of the flow rate, but insisted the predictions had not affected the response.

"This has been our highest priority since the crisis occurred," Obama said, insisting his administration was fully responsible for directing the relief effort. "In case you were wondering who is responsible? I take responsibility".

A total of anywhere from 260,000 barrels to 540,000 barrels of oil has spewed out of the well since April 20, McNutt said. That would make it bigger than the estimated 257,000 barrels of oil that leaked from the Valdez off the coast of Alaska.

About half of the oil has already been burned, collected by skimming ships, or simply evaporated during the ongoing relief efforts, McNutt said.

Weeks of talk about top kill have built up expectations, but company officials warned against hopes for a quick fix. BP chief Tony Hayward has said the operation has a 60-70 per cent chance of success.