But BP boss Tony Hayward said the firm was not to blame for the accident which sank the Deepwater Horizon rig on 22 April, causing the slick. He said the equipment that failed belonged to drilling firm Transocean.

Thousands of barrels of oil have been leaking into the Gulf of Mexico every day since the rig sank. Some oil has washed ashore, but officials say the bulk of the slick remains a few miles from the Louisiana coastline.

Both companies are expecting lawsuits over the slick, which threatens to cause major ecological damage.

US President Barack Obama has described the oil leak as a "potentially unprecedented" environmental disaster. "BP is responsible for this leak. BP will be paying the bill," he said.

The US government has been putting pressure on BP to act quickly.