BP has been able to force mud down into the oil leak in the Gulf of Mexico and so far they've halted hydrocarbons from coming up.
BP has been able to force mud down and not allow any oil to come back up.
The oil company is in a "wait and see" mode now to see how the well stabilizes.
President Obama has extended for six months a moratorium on offshore oil drilling in deep water.
BP has stopped the oil gushing out of a ruptured Gulf of Mexico well, U.S. officials said Thursday, but cautioned it was still too early to declare victory in the five-week disaster.
"They've been able to stabilize the wellhead, they're pumping mud down it. They've stopped the hydrocarbons from coming up," said Coast Guard chief Thad Allen, who is coordinating the US government's battle against the oil spill.
He told local radio WWL First News that BP "had some success overnight" but cautioned the British energy giant was "in a period of kind of wait and see right now where they see how the well stabilizes."
"So everybody is cautiously optimistic, but there is no reason to declare victory yet. We need to watch it very, very carefully."
The news came after BP yesterday launched a maneuver dubbed a "top kill" in a bid to plug the leak which has been gushing oil into the Gulf since a April 20 explosion ripped through the Deepwater Horizon drilling platform.
After at least two failed attempts to cap the spill, BP has come under increasing pressure from the US administration and furious residents helplessly watching oil wash ashore, with 100 miles (160 kilometers) of Louisiana coastline now contaminated.
President Barack Obama moved Thursday to extend a moratorium on deepwater oil drilling for six months.
He was to take questions at a press conference over the government's handling of the spill ahead of his second visit to the region on Friday.
In a new development highlighting environmental problems, all 125 commercial fishing boats helping to clean up the oil off Louisiana's Breton Sound were recalled after four workers reported health problems.
The crew members aboard three separate vessels working in the area "reported experiencing nausea, dizziness, headaches and chest pains," the unified joint command center said.
It raised new questions about the risks of working with the thick gobs of oil washing up on shores here and the toxicity of tens of thousands of gallons of chemical dispersants used by BP to break up the slick.
After reviewing the disaster that sank the Deepwater Horizon rig, a White House aide said Obama would extend for six months a moratorium on offshore oil drilling in deep water.
He also will delay planned oil exploration projects off the coast of Alaska until after a review by a presidential commission.
Obama will also cancel plans to lease drilling rights in the western Gulf of Mexico and off the coast of Virginia, the aide said.
The work to cap the leak is being carried out by remote-controlled robotic submarines a mile below the surface.
It aims to drown the oil flow by dumping heavy drilling fluids on top, pushing back the oil long enough to dump cement on top and then seal it with cement.
After BP officials admitted there had been a litany of failures and warnings signs in the hours before the explosion, the New York Times reported that the company chose a casing for the well that was the riskier of two options, partly because it made "the best economic case."
Citing a BP document, the daily said the concern with the method BP chose was that if the cement around the casing pipe did not seal properly, gases could leak all the way to the wellhead, where only a single seal would serve as a barrier.
The other option under consideration would have provided two barriers in case of a gas leak, the Times said.
Rig workers and company officials have said gases were leaking through the cement hours before the explosion. .
Louisiana Governor Bobby Jindal said Wednesday some 100 kilometers of his state's coastline was now soiled, more than doubling the previous estimate. The oil is imperiling rare species of animal and plant life, and has also led to a major fishing ban in the marine rich waters.
A tour of coastal areas left the president of the local Plaquemines parish aghast at the devastation.
"The same oil that's been out there two weeks ago is still out there. And nothing is being done," Billy Nungesser told CNN Wednesday.
BP officials have also readied back-up options but some, including the drilling of relief wells to divert the flow and allow the original well to be capped, could take several months.