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.
Published Date: 2010-06-04 05:59:39 GMT