BP shares are expected to come under more pressure amid news that the US oil spillage crisis is getting worse.
As the slick headed for the US coastline, President Barack Obama said BP was "ultimately responsible for funding... clean-up operations".
The company's shares plunged 7% on Thursday when it admitted that oil was leaking at a far faster rate than first thought.
Some 5,000 barrels of oil a day are now pouring into the Gulf of Mexico.
That is five times as much crude than was thought to be spilling from under the Deepwater Horizon rig when it exploded last week.
BP said the clean-up operation was costing it millions of dollars a day, but this figure could rise dramatically if the oil hits land.
The oil, which has formed a slick measuring 45 miles by 105 miles, is currently about 50 miles (80km) off Louisiana's coast.
The US government has designated the Gulf of Mexico oil spill as an "incident of national significance". And the Louisiana has declared a state of emergency.
Speaking at the White House, Mr Obama said: "While BP is ultimately responsible for funding the cost of response and clean-up operations, my administration will continue to use every single available resource at our disposal, including potentially the Department of Defence, to address the incident.
"And I have ordered the Secretaries of Interior and Homeland Security, as well as Lisa Jackson of the Environmental Protection Agency to visit the site on Friday to ensure that BP and the entire US government is doing everything possible, not just to respond to this incident, but also to determine its cause."
US Homeland Security Secretary Janet Napolitano said she wanted the "strongest possible response" from BP.
Weather forecasters have warned that changing winds could drive the oil slick ashore on Friday, and the US Defense Department has called in more military resources to help.
President Obama's press secretary Robert Gibbs said no one was blaming BP, but added: "Under the Oil Pollution Act, BP pays for all this".
In an email to BP staff, the company's chief executive, Tony Hayward, called the explosion, in which 11 rig workers are missing and presumed dead, "a tragic disaster".
On Thursday, the chairman of the House of Representatives' Select Committee on Energy Independence and Global Warming wrote to BP and four other oil giants requesting executives testify at a hearing to be held into the disaster.