BP has begun an operation to fit a tighter cap over its blown-out oil well in the Gulf of Mexico.
An underwater robot has taken off the existing dome to make room for the new sealing cap.
But the operation may last between four and seven days - meaning oil will flow unimpeded into the sea.
Another system is also being linked up and may start collecting crude on Sunday, BP says. It is drilling two wells to intercept and block the leak.
It estimates the first of these wells will be finished in the first half of August, enabling the company to intercept fully the damaged well.
The explosion of the Deepwater Horizon rig in April killed 11 people. President Barack Obama has called the leak the biggest environmental disaster in US history.
Increased potential
BP's submersible robots have removed the current containment cap from the ocean floor, but now it will be several days at best before the new cap is in place.
A fleet of 400 skimmers is in place to try to minimise the effects of increased oil loss during the operation, BP said.
"It's not just going to be, you put the cap on, it's done. It's not like putting a cap on a tube of toothpaste," Coast Guard spokesman Capt James McPherson warned.
The new sealing cap - similar to a smaller version of the damaged blow-out preventer - has three devices that are designed to shut the flow.
According to BP, the new cap "creates the potential to increase oil and gas containment capacity to greater than 50,000 barrels per day and should improve containment efficiency during hurricane season by allowing shorter disconnect and reconnect times".
The company said that "there can be no assurance that the sealing cap will be successfully installed or installed within the anticipated timeframe", given it has never been attempted at this depth before.
It said that contingency caps were at the ready on the seabed should the replacement operation fail.
The company also said that favourable weather conditions had made it possible to begin another operation in tandem - connecting a new ship to the blow-out preventer.
The ship is known as the Helix Producer and it will begin collecting oil and sending it to a third ship being used for the purpose.
BP says this could begin "ramping up containment operations as early as Sunday".
The containment dome currently collects about half the amount of oil gushing out of the damaged well.
On Friday, the Coast Guard commander overseeing the response, Adm Thad Allen, said the flow of leaking oil could be contained by Monday. But in that time hundreds of thousands of barrels of oil could flow unabated into the ocean.
"We have a significant chance to dramatically reduce the oil that's being released into the environment and maybe shut the well in altogether in the next week," he said.
"I use the word 'contained'," said Adm Allen. "'Stop' is when we put the plug in down below."
At the moment it is believed that BP is siphoning off only around half the leaking oil.
Current US government estimates of the spill range from between 35,000 to 60,000 barrels a day.
On Thursday BP said its operation to drill a new relief well to stop the Deepwater Horizon leak was ahead of schedule.
BP's bill so far has been more than $3.1bn - and it has agreed to set up a $20bn fund to deal with compensation claims and clean-up costs.
