Sea birds covered in spilt oil are being treated in Louisiana
Oil spilling out of a ruptured pipeline in the Gulf of Mexico may reach the southern Florida coastline, a top coast guard official has told the US Senate.
Any oil that does wash up is likely to be in the form of tarballs which are a "little easier to manage", Rear Adm Peter Neffenger said in testimony.
Reports from the White House say a presidential commission will be set up to investigate the disaster.
And a top official who oversaw offshore drilling has announced he is retiring.
Chris Oynes said he would step down at the end of this month.
There was no official comment on the reasons for the resignation of Mr Oynes from the Minerals Management Service (MMS). Senior officials there have been accused of becoming too close to the industry.
Oil has been spewing into the Gulf since BP's Deepwater Horizon rig exploded on 20 April.
US President Barack Obama has described it as a "potentially unprecedented" environmental disaster.
An unnamed Obama administration official told the Associated Press news agency that President Barack Obama would establish a commission by executive order.
Tarballs of oil are said to be more manageable
It would, the official said, be similar to panels created to investigate the space shuttle Challenger disaster and the nuclear accident at Three Mile Island.
It would study oil industry practices, rig safety, regulation and governmental oversight, as well as the "structure and functions" of the MMS, an official told Reuters news agency.
At the Senate Committee on Homeland Security and Governmental Affairs, Rear Adm Peter Neffenger said that the government was closely watching whether the oil would be swept up into the "loop current" that moves around Florida.
"Currently it shows to be somewhere in the neighborhood of 40-50 miles [65km-80km] from the southern edge of the spill," he said.
"We are watching that carefully and as a result of that we are preparing for potential impact on the southern Florida coast and impacts around the southern Florida coast."
Tarballs would, he suggested, be "a more manageable piece" than the slick in the Gulf of Mexico.
Homeland Security Secretary Janet Napolitano also appeared at Monday's hearing, the first time senators were able to question officials about the response to the 20 April disaster.
She said the government was monitoring the loop current very closely and were treating it as if it were a coastline.
"In other words... if we were to see that the oil really was beginning to move toward the loop current we would begin doing some things in the way of dispersant and booming... as if the loop current itself were a piece of the coast," she said.
Senator Joseph Lieberman, who chairs the committee, said at the hearing that the government should not allow any new deep water wells to get permits or be drilled until the industry could prove it can prevent another failure of drilling equipment in deep waters or contain oil spills more effectively.
"I say that with regret because I know how important offshore American oil is to our nation's energy independence," he said.
"But the US government has a responsibility to the public safety that is more important and that responsibility, I fear, was not fulfilled in this case."
BP's America president, Lamar McKay, returned to the Senate for the hearing, having already faced questions along with other oil industry executives last week.
Protesters greeted Mr McKay at the Senate with placards
Scientists said on Sunday they had found vast underwater plumes of oil, one 10 miles (16km) long and a mile wide, lending weight to the fears of those who believe the actual spill could be many times greater than the estimate of 5,000 barrels daily.
But Mr McKay told senators: "I think we ought to be cautious in terms of defining what plumes are out there and how they're behaving."
The US National Oceanic and Atmospheric Administration cast doubt on the plume reports, saying no definitive conclusions had been reached because the findings were unverified.
BP said earlier on Monday it was managing to funnel the equivalent of 1,000 barrels a day of oil from the blown well to a tanker ship with the use of a mile-long tube.
That would amount to a fifth of the estimated daily spill of 5,000 barrels - an estimate made by the Coast Guard and BP.
"This is just containing the flow, later this week, hopefully before the end of the week, we'll make our next attempt to actually fully stop the flow," BP Chief Operating Officer Doug Suttles told NBC's Today programme.
BP finally managed to insert the tube into the leaking pipe, using underwater robots, on Sunday at the third attempt.
But, in a joint statement, Ms Napolitano and Secretary of the Interior Ken Salazar said the latest technique was "not a solution to the problem and it is not yet clear how successful it may be".
BP also said on Monday it had received 15,000 claims for compensation, and had already paid out on 2,500 claims. Experts warn that BP's total liability for the spill could run into billions of dollars.