Under mounting pressure to contain the month-long oil spill in the Gulf of Mexico, British oil major BP on Tuesday morning outlined its next steps to stop the undersea leak.

The first option is still the so-called 'top kill' operation where heavy drilling fluids would be injected into the well to stem the flow of oil and gas and, ultimately, kill the well, BP said in a press release.

Most of the equipment is on site and preparations for the "top kill" operation continue, with a view to deployment within a few days, it said.

Earlier, BP officials said they might start the procedure as early as Wednesday morning.

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.

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.