Oil firm BP has lowered a cap on to a leaking oil well in the Gulf of Mexico, in the hope of piping some of the oil to ships on the surface.
Video footage showed the cap lowered into place against pressure from escaping oil and gas.
BP said some oil had started flowing up the pipe attached to the cap, but it could take the rest of the day to determine how much.
The US Coast Guard said the cap would only be a temporary, partial fix.
Live underwater video footage from the site on Friday showed quantities of oil still escaping into the sea.
BP's latest attempt to cap the well followed mounting criticism from US President Barack Obama.
He told CNN that he had not seen "the kind of rapid response" to the disaster that he would have liked and that BP had already "felt his anger" over the spill.
He said was "furious at this entire situation".
Mr Obama has for the second time postponed a trip to Australia, Indonesia and Guam, in order to deal with the crisis.
He is due to visit the gulf later on Friday - his third trip to the area since the leak began six weeks ago.
BP's share price rose 4% in early European trading.
Chief executive Tony Hayward is to hold a conference call with investors, which will be available to the public on the company's website, from 1400 BST (1300 GMT).
Mr Hayward is expected to try to reassure investors.
BBC economics correspondent Andrew Walker says one key question is whether BP will pay shareholders its dividend as normal.
He says if it does, this will go down very badly in the US, where pressure is mounting for BP to retain the cash for the clean-up.
Mr Hayward has said the results of the latest capping effort should be known later on Friday.
BP said it would need 12-24 hours of readings to determine whether or not the cap was working because of the uneven flow of oil and gas from the leak.
Chief operating officer Doug Suttles said on Friday: "There is flow coming up the pipe. Just now, I don't know the exact rate."
BP has said it does not expect to be able to fully halt the oil flow until August, when two relief wells will be completed.
In the first phase of the latest effort to contain the spill, robots cut a leaking pipe.
BP then used giant shears manipulated by undersea robots to snip off the end of the pipe, after a diamond-edged saw failed to do the job.
A funnel-like cap has been placed on top, which if successful, would improve oil recovery and BP would hope to collect the oil on a surface ship above the well.
The US Coast Guard said the placement of the cap was a positive development, but that it would be "some time before we can confirm that this method will work and to what extent it will mitigate the release of oil into the environment".
Mr Hayward acknowledged that what engineers were doing had never been tried before, and said there were many challenges ahead.
The White House has been under increasing pressure to show the administration is in control of the response and clean-up efforts.
The government said on Thursday it would send a $69m (£47.1m) bill to BP for expenditure incurred from its response to the spill.
BP estimates that the disaster has so far cost the company about $990m.
A "top kill" procedure, which had been considered the best hope for plugging the leak, failed over the weekend.
The Deepwater Horizon drilling rig, leased to BP, exploded and sank in April, killing 11 workers.