Oil company BP says it has cut a ruptured pipe from the leaking Gulf of Mexico oil well, a key step in the latest effort to cap the well.
BP chief Tony Hayward hailed it as an "important milestone" and said BP could know within the next 12 to 24 hours if the capping effort would succeed.
For now, undersea robots are preparing the well-head to receive the cap.
President Barack Obama is to visit the gulf on Friday, his third trip there since the leak began six weeks ago.
The White House has been under increasing pressure to show the administration is in control of the response and clean-up efforts.
The federal government will send a $69m (£47.1m) bill to BP Plc on Thursday for expenditure incurred from its response to the spill, the White House said.
Meanwhile, a detailed computer modelling study indicated that oil from the spill might extend along thousands of miles of the Atlantic coast and open ocean as early as this summer.
The National Center for Atmospheric Research said once oil entered the "Loop Current", part of the Gulf Stream which sweeps around the Florida panhandle, it would be only weeks before it reached Florida's Atlantic shores.
Speaking in Houston, Mr Hayward described the pipe-cutting procedure as "simply the beginning".
BP 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.
The challenge is now to place a containment cap securely over the cut and stop the oil flow. The company hopes to collect the oil on a surface ship above the well.
"Our task is to contain the oil, ultimately to eliminate the leaking well, and most importantly, to clean up all of the oil, defend the shoreline and restore the shoreline where oil does come ashore such that we return it to its original state," Mr Hayward said.
BP estimates that the disaster has so far cost the company about $990m in clean-up costs.
Two Democratic senators have written to Mr Hayward urging him to suspend payments to shareholders worth $10bn until all costs of the clean-up and compensation are paid out.
On Wednesday, BP said it would pay for the construction of six sand barriers off the coast of the US state of Louisiana.
The sand barrier project will push BP's bill to about $1.4bn.
BP share prices have continued to plummet in trading on the London Stock Exchange, amid news the US justice department has opened several civil and criminal inquiries into the Gulf spill.
Mr Obama has used the disaster to urge Congress to pass a bill to overhaul US energy policy and end tax breaks for oil companies.
The oil began leaking into the Gulf of Mexico on 20 April when the Deepwater Horizon drilling rig, leased to BP, exploded, killing 11 workers.
BP is drilling two relief wells to permanently stop the leak but they are not expected to be completed until August.
A "top kill" procedure, which had been considered the best hope for plugging the leak, failed over the weekend.
Engineers were unable to pump enough heavy mud into the well to staunch the oil flow.