The news came as the White House said it will send the energy giant an initial bill of $69 million to pay for the damage caused by the leak, and subsequent clean-up efforts.

Hayward said BP is committed to cleaning up the spill, the largest in US history. "We will meet our obligations to all stakeholders," he said.

Step forward

Retired Coast Guard Admiral Thad Allen, the US disaster response chief, called the  underwater developments a "significant step forward."

in depth

On Wednesday, the giant oil slick was spotted a matter of miles from the pristine tourist beaches of the Florida coast, prompting fears that it will wash ashore in a fourth US state.

So far more than 200km of Louisiana coast have been contaminated, triggering long-term fears for the region's already vulnerable coastal wetlands and native wildlife, including lucrative fishing grounds.

Scientists from the University of Miami released a study on Wednesday showing the oil slick's surface area had expanded to cover 24,435km sq of the Gulf - triple the size of satellite imagery from May 1.

Experts say the best chance of stopping the leak once and for all remains the drilling of a relief well, but that is still at least two months from completion.

Criminal investigation

The US government, which has launched a criminal investigation into the disaster, has estimated the flow of oil to be around 12,000 to 19,000 barrels a day - meaning more than 75 million litres have poured into the Gulf since April.

In a speech on Wednesday, Barack Obama, the US president, said the Gulf oil spill had highlighted the risks associated with oil drilling and called on the US to break its dependence on fossil fuels.

"An America run solely on fossil fuels should not be the vision we have for our children and grandchildren," he said at Pennsylvania's Carnegie Mellon University.

Obama said a price must be put on carbon pollution, and pledged to make a new push for the energy bill that has been stalled in the senate.

"The votes may not be there right now, but I intend to find them in the coming months," he said. The US president will visit the Gulf coast again on Friday to meet those affected by the spill, which he has called the worst environmental disaster in American history.