BP has said the amount of oil recovered from a leaking Gulf of Mexico oil well on Thursday was less than half an earlier estimate.
BP said it had siphoned 2,200 barrels in the 24-hour period to midnight on Thursday, down from an estimate of 5,000 barrels earlier in the day.
The US government has formed a team to develop a more precise estimate of the amount of oil gushing from the well.
BP will attempt to stem the oil flow next week, instead of Sunday as hoped.
The oil leak began more than a month ago, when a rig leased by BP exploded.
The spill has reached Louisiana and is threatening Florida and Cuba.
Thick, sticky oil is washing on to miles of fragile Louisiana wetlands, with brown, foul-smelling globs coating reeds and grasses.
The crude oil has been flowing since the offshore drilling rig Deepwater Horizon exploded off the coast of Louisiana on 20 April and sank two days later.
'Flow not constant'
It was widely expected that BP would proceed on Sunday with a so-called "top kill" operation to plug the well, which underwater video shows to be gushing oil and gas.
But BP chief operating officer Doug Suttles on Friday said the move would be delayed until next week, perhaps on Tuesday.
In the "top kill" operation, heavy mud would be injected to stem the oil flow, then cement used to block the well permanently.
The delay was announced on the same day the company indicated the amount of oil it has been able to capture from the well was significantly less than initially estimated.
A BP spokesman told Reuters news agency on Friday that the company had only siphoned off 2,200 barrels, or 92,400 gallons, in the 24 hours up to midnight on Thursday.
"The flow changes, it's not constant," BP spokesman John Curry said on Friday.
But on Thursday, a spokesman had said the company was collecting 5,000 barrels per day or more.
A live video feed showing the oil gushing from the well has been made available by BP after pressure from a US congressman.
It shows a large, flowing plume of oil and gas next to the tube that is carrying some of it to the surface.
It remains unclear exactly how much oil is streaming from the burst well 5,000ft below the surface, and the US government has formed a multi-agency task force tasked with precisely gauge the amount.
Independent scientists have estimated the flow is as much as 10 times more than the widely distributed figure of 5,000 barrels, or 210,000 gallons, per day.
In the meantime, the Obama administration has asked BP to make public all measurements of the growing leak, air and water quality samples, trajectories of underwater plumes and locations of dispersants.
The request came in a letter to BP chief executive Tony Hayward from Homeland Security Secretary Janet Napolitano and Environmental Protection Agency (EPA) administrator Lisa Jackson.
It said: "In responding to this oil spill, it is critical that all actions be conducted in a transparent manner, with all data and information related to the spill readily available to the United States government and the American people."
The officials said that despite claims by BP it was making efforts to keep the public and the government informed, "those efforts, to date, have fallen short in both their scope and effectiveness".
They said it was "imperative that BP promptly" make public all data on the spill.
Dispersant damage warning
Also on Friday, Louisiana Governor Bobby Jindal demanded the federal government and BP commit to long-term clean up efforts until the tainted beaches and marshlands have been restored to their pre-spill state.
And a top coast guard official in Louisiana speculated that oil had washed ashore because the weather had been too calm for the past two days to use chemical dispersants to mitigate the slick.
The EPA has told BP to use a new dispersant to contain the spill, as fears grow about potential damage from the use of such chemicals.
BP has used a number of dispersants, all approved by the EPA, to try to break up the spill.
It emerged on Wednesday that a small part of the slick had entered the so-called Loop Current, which could take it to Florida and up the eastern US coast.
Florida forecasters said it would be at least a week before the oil reached waters near the state.
Farther south, US officials have been talking to Cuba about how to respond should the spill reach the island's northern coast.