US officials have said the cap placed by engineers over a leaking oil well in the Gulf of Mexico is now capturing about 1.7 million litres of oil a day.
But estimates on the amount of oil being caught range from one-third to three quarters of the total volume that continues to gush into the sea.
Live video from the site of the leak late on Monday showed dark clouds of oil escaping around the cap.
On Monday oil giant BP said it expected to increase collection "over the next few days", adding that its engineers planned to replace the cap collecting the crude with a slightly bigger device next month.
The newer cap will "provide a better, tighter fit" than the current one, BP spokesman Robert Wine told The Associated Press.
But it will also allow the oil now being collected to again spew out into the Gulf during the changeover.
Meanwhile the US government's point-man for the disaster has said that efforts to clean up the oil are becoming increasingly complicated as the massive slick breaks up into hundreds or even thousands of patches that could inflict damage for years.
"Because what's happened over the last several weeks, this spill has disaggregated itself," Coast Guard Admiral Thad Allen said.
"We're no longer dealing with a large, monolithic spill. We're dealing with an aggregation of hundreds or thousands of patches of oil that are going a lot of different directions."
Dealing with the oil spill on the surface would probably last "for a couple of months", Allen said, but he added that the "long-term issues of restoring the environment and the habitats and stuff will be years."
The oil began gushing after an explosion sank the BP-operated Deepwater Horizon oil rig on April 20.
|Obama has said that BP should compensate all those affected by the spill [EPA]
The flow of oil recently increased in volume after engineers cut off the top of the damaged outflow pipe as part of the latest containment effort.
BP believes the bigger cap it plans to deploy next month will fit over more of the pipe than the one currently in place.
The energy giant is also moving to siphon more oil through other seabed equipment.
By later this week, BP said, engineers intend to start reusing a setup that pumped heavy drilling fluid into a failed blowout preventer in last month's unsuccessful "top kill" attempt to plug the leak.
That system will pull oil and gas through the same equipment and channel it to the same service rig at the surface that was used for the top kill, BP said.
The slick from the ruptured pipe has stained beaches and marshes in spots along more than 160km of coast from Louisiana to Florida and a sheen on the sea surface has been spotted as far as about 240km west of the city of Tampa in Florida.
On Monday, Barack Obama, the US president, again ramped up pressure on Bp to do more to respond to the disaster, saying he believed the London-based oil giant should compensate all those affected by the spill.
|Scientists have warned that the damage from the spill may last for years [AFP]
In an interview with broadcaster NBC he also hit out at those who have criticised his administration's own handling of the crisis, saying he was fully engaged with what was happening and doing all that was possible.
"I was down there a month ago before most of these talking heads were even paying attention to the Gulf," he said.
"I don't sit around just talking to experts because this is a college seminar, we talk to these folks because they potentially have the best answers so I know whose ass to kick."
Obama said he would have fired Tony Hayward, BP's chief executive officer, over his handling of the oil spill crisis.
Earlier he sought to reassure Americans, saying that "we will get through this crisis" but that it would take dedication.
"This will be contained. It may take some time, and it's going to take a whole lot of effort," he said.
"There is going to be damage done to the Gulf Coast, and there is going to be economic damages that we've got to make sure BP is responsible for and compensates people for."