A top US Coast Guard official has said that BP's plans to contain oil gushing into the Gulf of Mexico from a blown-out well do not go far enough.
Rear Adm James Watson gave the oil firm 48 hours to identify "additional leak containment capacity".
The warning follows new US government data suggesting the flow of oil could be double previous estimates.
Speaking to UK PM David Cameron by phone, President Barack Obama said his criticism of BP was not anti-British.
In a letter to the oil firm, Rear Adm Watson said: "Because those estimates have now been revised and estimate a substantially higher flow of oil from the Macado 252 well, it is clear that additional capacity is urgently needed."
The letter was dated 11 June and released on Saturday.
"BP must identify in the next 48 hours additional leak containment capacity that could be operationalised and expedited," he said.
"I am concerned that your current plans do not provide for maximum mobilisation of resources to provide the needed collection capacity consistent with revised flow estimates."
The letter was a response to BP plans to contain the spill in a multi-phase operation taking a number of weeks.
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 11 workers.
A team of scientists put together by the US government and co-ordinated by the US Geological Survey has estimated that 20-40,000 barrels of oil a day were leaking into the Gulf of Mexico before a containment cap was placed on 3 June.
BP has said the device was collecting about 15,000 barrels of oil a day last week.
Last month, BP was estimating the leak to be the equivalent of just over 5,000 barrels per day.
BP's chairman has been asked to meet Barack Obama next week, amid assurances from the UK and US that bilateral ties will not be affected by the crisis.
In a telephone call with British Prime Minister David Cameron on Saturday, Mr Obama said criticism of BP over the oil spill has "nothing to do with national identity".
A statement from Mr Cameron's office said: "The president made it clear that he had no interest in undermining BP's value."
Mr Obama has referred to BP by its former name, British Petroleum.
Some British businessmen have accused the US of "anti-British" language following Mr Obama's blunt criticism of the firm.
BBC business editor Robert Peston has said BP is likely to bow to US pressure and suspend dividends to shareholders.
The oil giant has confirmed its directors will meet on Monday to discuss the possibility.
