BP Plc said its high-stakes bid to plug a gushing Gulf of Mexico oil well was proceeding to plan yesterday, while US President Barack Obama fended off criticism he has been too slow to respond to the worst oil spill in US history.

Nearly 40 days into the disaster, the BP oil leak has now eclipsed the worst previous spill, the Exxon Valdez wreck on the Alaska coast in 1989, according to new US government estimates of the amount of oil spewing into the Gulf.

BP's "top kill" procedure launched on Wednesday could choke off the well a mile (1.6 km) down on the sea floor, but BP said it needs another 24 hours or longer to know the outcome.

A day before he visits the Gulf for the second time, Obama vowed to hold BP accountable, saying was "angry and frustrated" at the London-based company's failure to plug the leak.

"In case you're wondering who's responsible, I take responsibility," he told a White House news conference. "It is my job to make sure everything is done to shut this down."

He announced a six-month extension of a moratorium on permits for new deepwater oil drilling while a commission investigates the causes of the disaster, and ordered 33 deepwater exploratory wells in the Gulf of Mexico to suspend work while they meet new safety requirements.

The move is a setback to offshore exploration and a potential blow to Obama's efforts to rejig US energy policy.

The political fallout from the spill also claimed its highest-profile victim as the head of the government agency that oversees offshore oil drilling resigned.

All eyes are on the top kill, the pumping of heavy fluid into the well to stop the gusher before capping it with cement. BP stopped pumping to analyze data, but resumed later Thursday, said BP chief operating officer Doug Suttles.

"We have not yet stopped the flow, so the operation has not yet achieved its objective," said Suttles at a briefing, adding that "nothing has gone wrong."

TAKING AN EMOTIONAL TOLL

News of progress on plugging the spill, coupled with a nearly 4 percent rise in oil prices, lifted shares of the companies most closely tied to the spill. BP shares gained nearly 6 percent in London trading.

Nervous investors have wiped about a quarter, or US$50 billion, off the energy giant's market value since April.

Millions of gallons of oil have poured into the Gulf since an April 20 blast on the Deepwater Horizon rig that killed 11 workers and triggered the spill, soiling 100 miles (160 km) of coastline, threatening some of the country's richest fisheries and endangering a fragile ecosystem already battered by Hurricane Katrina in 2005.

Weather could make matters worse. The US government forecast on Thursday that the Atlantic storm season that starts June 1 may be the most intense since 2005. Hurricanes could push more oil onto Gulf shores.

The destruction of both the local economy and a way of life was taking an emotional toll, even on elected officials. Louisiana Congressman Charlie Melancon broke down crying as he spoke to a House subcommittee meeting in Washington.

Locals who saw President George W. Bush's adminstration fumble in response to Hurricane Katrina were not impressed by Obama's pledges on Thursday.

"They ought to get the hell out of the way," said Bill Butler, who owns the Venice Marina, a hub for the now paralyzed sport fishing industry. "They can't even run the country. Just give us the equipment and get out of the way. The people of south Louisiana know what to do."

At the White House, Obama repeatedly defended his administration against mounting criticism from Gulf Coast residents that it was too slow to respond to the spill and too dependent on BP for solutions.

The spill could turn into a major political liability for Obama ahead of November elections widely expected to erode his Democratic Party's control of the US Congress.

"Those who think that we were either slow on our response or lacked urgency don't know the facts," said Obama.

"This has been our highest priority since this crisis occurred," he said.

KEY ENERGY OFFICIAL RESIGNS

Obama said federal authorities had been in charge of the response to the disaster since day one, but had been forced to rely on BP's superior underwater technology.

He also announced that planned exploration off the coast of Alaska would be suspended pending the presidential commission's review, while pending or proposed lease sales in the Gulf of Mexico and off the coast of Virginia would also be scrapped.

Earlier, Interior Secretary Ken Salazar said the head of the Minerals Management Service, Liz Birnbaum, had resigned. Many US lawmakers and environmental groups have blamed the federal agency for lax regulation of offshore drilling rigs and production platforms.

The scale of the Gulf of Mexico spill expanded hugely with new government figures on Thursday that put the flow rate from the ruptured well at as much as four or five times BP's estimate of 5,000 barrels (210,000 gallons/795,000 liters) a day.

US Geological Survey Director Marcia McNutt said various government teams examining the oil spill estimated the flow ranges from 12,000 barrels (504,000 gallons/1.9 million liters) to 25,000 barrels (1.05 million gallons/3.97 million liters ) per day.

The team's best estimate is 12,000-19,000 barrels per day, though one of three measurement methods the team used returned a high-end rate of 25,000 barrels per day, said McNutt.

BP declined immediate comment on the estimates.

In the previous worst US oil spill, in March 1989, the tanker Exxon Valdez hit an undersea reef in Prince William Sound, Alaska, spilling nearly 260,000 barrels (10.92 million gallons/41.34 million liters) of oil into the sea.

"It's as if two Exxon Valdez tankers have already run aground and more are on the way if they don't get this hole plugged," said Jeremy Symons of the National Wildlife Federation, adding that the USGS numbers showed BP could not be trusted.