There was mounting evidence on Tuesday night that the scale of the oil spill disaster in the Gulf of Mexico has grown beyond all the initial worst-case scenarios, as thousands of gallons of oil continued to gush from the sea floor.

In Key West, coastguard officials said about three tar balls an hour were washing up on the beaches at a state park at the southernmost point of the Florida Keys.

Such evidence suggests the damage wreaked by the spill — which began with an explosion on BP's Deepwater Horizon rig on April 20 — could now grow larger, with crude oil caught up in the powerful loop current that travels a much wider course through the Gulf and up the Atlantic coast. In response to the tar sightings, Washington doubled the no-fishing zone to 19 per cent of the waters in the Gulf.

The Obama administration admitted on Tuesday that it had underestimated the risks of offshore drilling. In a highly charged hearing in the Senate, Ken Salazar, the interior secretary, conceded failures in oversight by the agency responsible for policing offshore drilling. “We need to clean up that house,” he told the energy and natural resources committee.

The administration was for the first time held to account by Congress for the rigour of the Minerals Management Service (MMS), its regulatory body for offshore drilling. The agency was notorious in the George Bush era for sex-and-cocaine fuelled parties in Colorado.

Earlier, the oil spill disaster claimed its first resignation as a veteran official from the MMS brought forward his retirement. Mr. Salazar, under heated questioning from some Senators, was forced to concede that the agency had not been entirely cleansed in the 15 months under his charge. “We need to have the right regulatory regime in place and we will work hard to make sure that happens,” he said.

He admitted that the disaster had been a “wake-up call” and had persuaded him that policing of safety and environmental regulations on offshore oil rigs may have been inadequate. “My initial read on that is there should be additional safety requirements,” he told the committee.

Mr. Salazar also conceded there were “a few bad apples” among the inspectors of the MMS, and promised that if they over-ruled environmental advice from other government agencies — as alleged by some senators — they would be punished. “If there is someone in the department who ignored the science, then heads will roll,” he said.

But Mr. Salazar was adamant that the administration had been right to seek an expansion of offshore drilling last March, and made it clear there would be no revisiting that decision.

“The reality of it is we will be depending on oil and gas in the transition to a new energy future,” he said. He also refused multiple requests to provide a firm estimate for the size of the spill.

The focus on the administration's actions before the disaster is inconvenient for the White House, which this week intensified its efforts to limit the potential political damage on November's mid-term elections. Senators have also stepped up their pressure, with Barbara Boxer, the head of the environment and public works committee, demanding a criminal investigation into the spill, and pressing BP to produce video of the ruptured well. BP said it would comply.

But such defensive actions by the administration did not entirely shield Mr. Salazar and other officials of the Interior Department from the Senators' wrath. Senator Robert Menendez, a New Jersey Democrat and long-time opponent of drilling, said: “MMS has dramatically underestimated the potential risks here.” For the past 48 hours, administration officials have resisted reports by scientists that the spill could have entered the loop current or downplayed its significance. “By the time the oil is in the loop current, it's likely to be very, very diluted. And so it's not likely to have a very significant impact. It sounds scarier than it is,” said Jane Lubchenco, the head of the National Oceanic and Atmospheric Agency. On Tuesday, Ms Lubchenco took the decision to double the no-fishing zone. — © Guardian Newspapers Limited, 2010