US House Speaker Nancy Pelosi has accused BP of a "lack of integrity" when making its case for deepwater drilling in the Gulf of Mexico.
US President Barack Obama has met relatives of the 11 workers killed in the explosion on the BP-leased oil platform in the Gulf.
BP shares in the UK have fallen to their lowest level since 1997 amid fears of huge US penalties.
The US attorney general said America would "not pay a dime" for cleaning up.
BP would be held responsible for all damages, Eric Holder said.
In another development, the Obama administration said BP had agreed to quickly meet the compensation claims of businesses and individuals whose livelihoods had been disrupted by the spill.
Amid concerns in the UK over what some have described as growing anti-British rhetoric, the US state department insisted relations with America's "closest ally" would not be hurt by the oil spill.
Mr Obama has himself come under mounting political pressure over his handling of the crisis.
Oil has been leaking into the Gulf since the Deepwater Horizon rig exploded on 20 April and sank off the coast of the US state of Louisiana, killing the 11 workers.
'Not a dime'
"It is clear that there was a lack of integrity on the part of BP when it came to what it told us about the adequacy of their technology, the sufficiency of blow-out prevention and the capacity to clean up," Speaker Pelosi said.
Addressing reporters at the White House, she agreed with calls for BP to suspend its dividend to ensure that compensation is paid to Gulf oil spill victims.
Mr Obama said he and congressional leaders had agreed that they should look at how to update US law to ensure victims of the Gulf oil spill were protected.
Pollution legislation was written before the industry had developed ultra-deep water exploration and ought be brought up to date to ensure the people of the Gulf "are made whole", he added.
Mr Obama met families of those killed in disaster at the White House on Thursday.
A man whose son was among the dead said the president had told the families he would not forget about them. Keith Jones, a lawyer from Louisiana, described the meeting as a "very positive experience".
Mr Jones said Mr Obama had played with his four-week-old grandson, whose father Gordon was an engineer on the oil rig.
Amid growing public anger in the US, Mr Obama is keen to show he is on top of the situation and will make his fourth visit to the region on Monday.
His administration has been steadily applying more pressure on BP, and the US justice department is considering legal action to make sure BP has enough funds to cover the damage and compensate those affected by the slick.
"I can make this pledge to the American people - that the American people will not pay a dime for the clean-up of the Gulf region and that BP will be held responsible for all the damages that have occurred," Mr Holder told reporters in Washington.
"We will take the necessary steps to make sure that that occurs."
Tracy Wareing, an official at the US National Incident Command office, said an understanding on expediting the payment of claims had been reached at a meeting with BP executives including CEO Tony Hayward.
Administration officials had raised a "pressing concern" about the time BP was taking to provide relief payments, particularly to businesses in the stricken area, she added.
BP says a containment cap system placed on the blown-out well last week collected 15,800 barrels of oil on Wednesday - slightly up on the 15,010 barrels collected in the previous 24-hour period.
The company has come under increasingly sharp attack by some US politicians for its handling of the spill, described as the worst environmental disaster the US has faced.
Shares in the British oil giant have nearly halved over the last couple of months.
The UK government on Thursday sought to play down fears expressed by some senior political and diplomatic figures of "anti-British rhetoric" in the US.
Prime Minister David Cameron, who will discuss BP with President Obama this week, said he understood the US government's "frustration".
US state department spokesman PJ Crowley said the spill was a matter between the US and BP, not the UK.
"BP is a private company and this is about the impact of a tragedy in terms of the explosion of the oil platform and the resulting oil spill," he told reporters.
"This is not about relations between the United States and its closest ally."