U.S. government scientists say three-quarters of the oil from BP's massive spill in the Gulf of Mexico has been cleaned up or broken down by natural forces.

White House energy adviser Carol Browner said on morning TV talk shows Wednesday that a new assessment found that about 75 per cent of the oil has either been captured, burned off, evaporated or broken down in the Gulf.

Browner welcomed news early Wednesday from BP PLC that it has plugged the broken well with heavy mud.

The oil giant called it a milestone in ending the worst offshore oil spill in US history that started with an April 20 drill rig explosion off Louisiana that killed 11 workers.

BP spokeswoman Sheila Williams says the "static kill" done Tuesday has reduced the pressure and the mud is holding the oil down. The US government still wants BP to do a "bottom kill" from a relief well still being drilled.