Oil giant BP faced mounting pressure on Monday to control a massive oil slick in the Gulf of Mexico after the US government threatened to take over the response to the month-old disaster.

"If we find that they're not doing what they're supposed to be doing, we'll push them out of the way," said Interior Secretary Ken Salazar.

Salazar insisted that the full force of the federal government was bearing down on BP, which is legally responsible for dealing with a ruptured pipe that has been gushing oil into the gulf from the wreckage of an offshore rig.

The BP-leased Deepwater Horizon rig exploded on April 20, killing 11 workers, and sank two days later. Ever since, hundreds of thousands of gallons of oil, perhaps even millions, have been spewing each day into the sea.

Salazar lashed out at BP for missing "deadline after deadline" as its latest attempt to cap the environmentally devastating leak was hit with further delays, saying the company faced an "existential crisis".