BP's complex "top kill" manoeuvre to plug its Gulf of Mexico oil well has failed, crushing hopes for an end to the largest oil spill in US history already in its 40th day.

It may be another two months before the oil giant can definitively turn off the gusher - a delay that could undermine US President Barack Obama as he faces growing criticism for a perceived slow response to the disaster.

"We will continue to pursue any and all responsible means of stopping this leak until the completion of the two relief wells currently being drilled," Obama said in a statement after the news, noting that the wells will take months to complete.

BP's Chief Operating Officer Doug Suttles told reporters the London-based firm would now attempt to capture oil from the well after cutting the pipeline and attaching a new cap.

However, Suttles warned the new procedure would take at least four days and would only capture "most" of the leaking oil.