US President Barack Obama has called on his party and supporters to back a "new future" of clean energy.

Visiting areas affected by the BP oil spill, he said the full resources of government were being used, with 17,500 National Guard soldiers available.

BP's US head, Lamar McKay, is to return to Congress shortly for further questioning.

Oil has been spewing into the Gulf of Mexico since a drilling rig it leased exploded and sank in April.

This week, the BBC is assessing the impact of the Louisiana oil spill. To see the scale of the new deep water drilling rigs in one of the world's biggest and deepest off-shore fields, join BBC World News in Brazil on Tuesday with Paulo Cabral. Full coverage of the oil disaster

The company has put the cost of the clean-up to date at $1.6bn (£1bn).

Mr Obama has been touring Mississippi, Alabama and Florida, on his fourth visit to the region since the spill.

He promised Americans living on the Gulf of Mexico that their beaches would be restored to pristine condition, vowing that he would bring full force to bear on BP to pay for the damage from oil pollution.

He is due to put his demands to BP executives when he meets them face to face for the first time on Wednesday.

On Monday, the company's share price once again fell sharply, dropping 9% in London and New York.

Directors also met to discuss whether to suspend dividend payments to shareholders, amid pressure from the US government.

BP placed a containment cap on its damaged oil well earlier this month; by last week, it was collecting about 15,000 barrels of oil a day.

The US Coast Guard estimates a daily leak of about 35,000 barrels.

'Environmental 9/11'

Mr Obama visited the Mississippi town of Gulfport before moving on to Theodore in Alabama.

"We are confronting the largest environmental disaster in our history, with the largest environmental response and recovery effort in our history," he said there.

MARDELL'S AMERICA The moral pressure on [BP] will only grow by the end of this week Mark Mardell, BBC North America editor Read Mark's thoughts in full

The government had held preliminary conversations with BP about how to deal with compensation claims and expected that a mechanism would be agreed soon, he added.

He announced measures to protect the Gulf seafood industry, and pledged that beaches in the area could and would be returned to their pristine condition within one or two years.

Earlier, in a statement aimed both at paid-up Democratic Party members and at millions of individuals who backed his 2008 presidential bid online, the president asked his network to lend their name to a campaign to change the way America produces and consumes its energy.

"Beyond the risks inherent in drilling four miles beneath the surface of the Earth, our dependence on oil means that we will continue to send billions of dollars of our hard-earned wealth to other countries every month - including many in dangerous and unstable regions," he said.

"In other words, our continued dependence on fossil fuels will jeopardise our national security. It will smother our planet. And it will continue to put our economy and our environment at risk.

"We cannot delay any longer, and that is why I am asking for your help."

His remarks followed comments in which he said the ongoing oil spill would change the way the US thinks about energy forever - comparing the effect of the disaster on the environment to the way 9/11 shaped US security policy.

Next steps

While Mr Obama has visited Louisiana a number of times since the start of the crisis, on his latest trip he was touring areas of the other affected states.

He was scheduled to spend the night in the region before returning to Washington on Tuesday.

On Tuesday night the president will make a prime-time TV address to the nation in which he is expected to outline the next steps his administration will take over the spill.

Speaking in Gulfport, Mr Obama said he empathised with those suffering financially from the oil spill, adding that the White House was still gathering facts and stories in order to press BP into awarding adequate compensation.

"There's still a lot of opportunity for visitors to come down here. There are a lot of beaches that have not been affected and will not be affected," the AFP news agency quoted him as saying.

"If people want to help, the best way to help is to come down here and visit."

Mr McKay is due to attend a hearing on Tuesday of the House of Representatives committee on energy and environment along with senior officials from other oil companies.

When he and other company industry heads testified before the Senate about the disaster last month, President Obama described the hearing as a "ridiculous spectacle". He talked of "executives of BP and Transocean and Halliburton falling over each other to point the finger of blame at somebody else".

BP's chief executive, Tony Hayward, who himself faces a House hearing on Thursday, has received a letter from the heads of two House committees which accuses BP of "taking shortcuts" on the design and construction of the well.

"It appears that BP repeatedly chose risky procedures in order to reduce costs and save time and made minimal efforts to contain the added risk," representatives Henry Waxman and Bart Stupak say.

The letter quotes internal e-mails between BP engineers before the disaster in which the site is described as a "nightmare well".