If necessary, BP said, it may combine "top kill" with the injection under pressure of bridging material to seal off upward flow through the blow out preventer.

If the "top kill" does not work, BP said it is considering placing another containment dome over the leak, possibly to be installed after several days.

Other options include lowering a second blow-out preventer, or a valve, over the leak, according to BP.

A worker cleans up the oil contaminated beach in Port Fourchon, Louisiana, the United States, on May 24, 2010.

At least 6 million gallons of crude oil have gushed into the Gulf of Mexico since the explosion and sinking of the Deepwater Horizon rig, which was leased by BP, in late April, according to a Coast Guard and BP estimate.

But some scientists believe the spill has already surpassed the 10.8 million gallons of oil spilled off Alaska during the 1989 Exxon Valdez oil spill, the largest oil spill to date in U.S. waters.

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