A boom is erected in Louisiana to counter the spill
The success of a move by oil giant BP to curb a leak in the Gulf of Mexico is "not clear" and the technique provides "no solution", the US has said.
It was responding to BP's move to siphon oil from the leaking well head to a tanker on the surface.
A government statement said it would not rest until the leak was permanently sealed and the spill cleaned up.
BP executive Kent Wells earlier said that the siphoning process was "working extremely well".
This was the third attempt it had made to insert a long narrow tube into the leaking pipe, using underwater robots.
It is thought that BP's 6in-wide (15cm) tube and stopper could capture more than three-quarters of the leak, although a smaller spill nearby also has to be contained.
Mr Wells said on Sunday at the firm's US headquarters in Houston, Texas, that over the next few days the company planned to slowly increase the amount of oil and gas flowing through the pipe to the tanker.
But a joint statement by Secretary of Homeland Security Janet 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".
It added: "We will not rest until BP permanently seals the well head, the spill is cleaned up, and the communities and natural resources of the Gulf Coast are restored and made whole."
It was the second strong statement to BP by the government officials on the current crisis.
Earlier they sent a letter saying they wanted to be sure BP would honour commitments not to limit costs to a US statutory cap of $75m (£50m).
BP responded by saying it had already made clear its position on paying damages for the disaster.
It said last week the cap was irrelevant and it would settle all legitimate damages claims.
"What they are requesting in the letter is absolutely consistent with all our public statements on the matter," said BP spokesman David Nicholas on Sunday.
Earlier, scientists said they had found vast underwater plumes of oil, one 10 miles (16km) long and a mile wide.
Researchers from the National Institute for Undersea Science and Technology said they had detected the slicks lurking just beneath the surface of the sea and at depths of 4,000ft (1,200m).
Samantha Joye, a marine science professor at the University of Georgia, said: "It could take years, possibly decades, for the system to recover from an infusion of this quantity of oil and gas.
"We've never seen anything like this before. It's impossible to fathom the impact."
Chemical dispersants BP has been dumping underwater may be preventing the oil from rising to the top of the ocean, the scientists said.
The find suggests the scale of the potential environmental disaster is much worse than previously feared since the Deepwater Horizon drilling rig blew up on 20 April, killing 11 workers.
Some scientists cast doubt on BP's estimate of the oil flow rate, saying the widely repeated figure of 5,000 barrels per day dramatically understates the real amount.
A week ago, BP tried to cap the well with a 100-tonne box, but gave up after it became encrusted with ice crystals.
Mississippi has become the third US state to have traces of oil wash up on its coast, along with Louisiana and Alabama.
The spill is threatening to eclipse the 1989 Exxon Valdez leak off Alaska as America's worst environmental disaster.