BP pumped heavy drilling mud into its blown-out Gulf of Mexico well on Tuesday in a "static kill" operation it hopes will help permanently plug the world's worst accidental marine oil spill.

The operation, aimed at subduing the still unstable deepwater gusher that was provisionally capped in mid-July, is part of a two-pronged strategy that seeks to finally seal the Macondo well later in August through a relief well.   

The "static kill" would take between 33 and 61 hours to complete. "The aim of these procedures is to assist with the strategy to kill and isolate the well, and will complement the upcoming relief well operation," British-based BP said in a statement.

Kent Wells, BP's senior vice president of exploration and production, told reporters in a briefing that drilling of the first of two relief wells, seen as the definitive, permanent "kill" solution, could resume by Thursday.

The relief well should intercept the ruptured Macondo well shaft in mid-August, so that more mud, and cement can be injected to shut it down for good.

The full magnitude of the Gulf of Mexico spill, triggered in April by a deadly rig explosion at the BP-owned Macondo well, became apparent on Monday as government scientists released revised figures showing almost 5 million barrels of oil leaked before the well was temporarily capped on July 15.

This made it the world's largest accidental maritime release of oil, surpassing the 1979 Ixtoc well blowout in Mexico's Bay of Campeche that gushed almost 3 million barrels.

Retired Coast Guard Admiral Thad Allen, who oversees the US oil spill response, said earlier that engineers had succeeded in halting a hydraulic leak found on Monday in the cap that could have hindered the "static kill" operation.   

Allen told reporters engineers were moving cautiously, initially pumping in only a small amount of mud, so as not to damage the fractured well and cause fresh leaks.

"This thing won't be truly sealed until those relief wells are done," Allen added.

The spill has unleashed an environmental and economic catastrophe on the Gulf Coast, disrupting the livelihoods of fishermen and tourism operators and triggering a barrage of damages lawsuits against BP. The company has said it will pay all legitimate claims and clean up fouled beaches and marshes.

The new leak estimates spelled further bad news for BP, which also faces an investigation by US securities regulators into whether its employees profited illegally from the spill.

The revised flow numbers suggest the company had underestimated costs by at least $1 billion.

BP had estimated the well had leaked some 4 million barrels of oil and that it would be fined $1,100 per barrel under the Clean Water Act. The company faces fines of $4,300 per barrel if gross negligence is proven, but said it saw no need to change its provision as a result of the new estimate. - Reuters