British Prime Minister David Cameron yesterday sought to ease anger over the worst oil spill in US history, as the energy giant crafted a new plan hoping to seal a blown-out well for good.

"The oil spill in the Gulf of Mexico is a catastrophe, for the environment, for the fishing industry, for tourism. I've been absolutely clear about that," Mr Cameron said after White House summit talks with US President Barack Obama.

The British leader stressed BP bore the responsibility for the clean-up operation with hundreds of miles of Gulf coastline contaminated, and thousands of lives wrecked by the damage done to the fishing and tourism industries.

BP has already spent close to $4 billion in clean-up costs and compensation claims, and Mr Cameron stressed it was in everyone's interests for the British energy giant to remain stable.

BP said yesterday it planned to sell assets in Pakistan to help cover the soaring costs, as part of a $10-billion sell-off announced last month.

"Equally, of course, BP is an important company to both the British and the American economies. Thousands of jobs on both sides of the Atlantic depend on it," Mr Cameron said.

"So it's in the interest of both our countries, as we agreed, that it remains a strong and stable company for the future."

The fate of the company is a sensitive issue, since BP's stock is the backbone of many British pension funds.

The US government has allowed BP to keep in place a cap stemming the flow from the ruptured wellhead for another 24 hours, as engineers float a new plan to kill the well.

BP said the aim would be to send down heavy drilling mud through the blowout preventer valve system that sits on top of the well and then inject cement into the wellhead to seal it.

"We're still very much in the design and planning phase," said senior BP vice president for exploration and production Kent Wells. "We've got some real experienced teams working on this over the next couple of days."

The latest plan is similar to a "top kill" bid at the end of May, weeks after the April explosion which tore through a BP-leased rig off Louisiana, when engineers spent days pumping drilling fluid into the leaking well.

That effort failed to smother the gushing crude, but officials believe the outcome could be different this time with the oil flow already contained.

US Coast Guard Admiral Thad Allen, who is overseeing the disaster response, said the planning was still in initial stages, and stressed two relief wells being drilled close to the busted well-head remain the ultimate fix.

US Energy Secretary Steven Chu told a meeting of major economies on clean energy that the Administration had not yet been briefed on BP's new plan and so had not taken a position.

Mr Chu, a Nobel laureate who has been consulted every step of the way on the disaster, said he sought "the ultimate goal of stopping the leak as quickly as possible while at the same time trying to minimise oil spillage."

Five days of "integrity tests" on the tighter-fitting cap placed on the damaged wellhead last week had detected seepage and other anomalies, Mr Allen said, but stressed they did not appear to be of major concern.

He ordered BP to produce a detailed timeline for restarting operations to contain the oil with surface vessels if there is any major leakage from the well-head and the cap has to be re-opened.

The announcement on Thursday that BP had stopped the oil flow completely for the first time since April raised hope among devastated Gulf communities that a three-month nightmare may soon be over.

The disaster began on April 20 when the Deepwater Horizon drilling platform exploded in the Gulf of Mexico, 80 kilometres off the coast of Louisiana, killing 11 workers.

The rig sank two days later rupturing the pipe that connected it to the well. Oil has since washed up on the coasts of all five Gulf states - Texas, Louisiana, Mississippi, Alabama and Florida.

It is not known exactly how much oil has leaked into the sea, but if the upper estimate of over four million barrels is confirmed, the disaster would be the biggest accidental oil spill ever.