There has been a fresh setback for BP in its attempts to contain the leaking Gulf of Mexico oil well, a company official says.
Initial efforts by the firm to insert a long tube into the broken pipe to funnel oil to the surface have failed.
A week ago, BP tried to cap the well with a 100-tonne box, but gave up after it became encrusted with ice crystals.
Mississippi has now become the third state to have oil wash up on its coast, along with Louisiana and Alabama.
Thousands of barrels of oil a day have been leaking from the seabed since the Deepwater Horizon drilling rig blew up, killing 11 workers on 20 April.
Early on Friday, BP began trying to thread a 6in (15cm) wide tube into a damaged 21in wide pipe on the ocean floor off Louisiana.
Like a long straw, the tube is intended to slurp oil to a ship on the surface, and a stopper surrounding it would stem the flow of crude into the sea.
But in a Saturday briefing in Robert, Louisiana, BP chief operating officer Doug Suttles said his technicians had hit a snag.
"We hope to have that tool inserted by sometime late tonight. It’s back on the seabed," he said.
"We did have to pull it back to surface yesterday to make some adjustments so we could connect it properly to the pipe-work that connects it to the ship, but we expect to begin operation of the equipment overnight."
Earlier, US Interior Secretary Ken Salazar told media at a Louisiana bird rescue centre BP had been forced to "reconfigure" in its attempts to insert the tube.
The oil giant has been using remote-controlled robots to guide the contraption to the seabed a mile beneath the surface.
BP chief executive Tony Hayward told the BBC’s Today programme in an interview broadcast on Saturday he hoped the leak could be stopped within 10 days.
But the worst case scenario was "more than that, and it’s impossible to say how much more", he added.
President Barack Obama warned on Friday there would no longer be a "cosy relationship" between oil firms and US regulators.
He condemned a "ridiculous spectacle" of oil executives shifting blame in US congressional hearings in recent days.
Mr Obama has stopped all new drilling for the moment and a number of politicians want that to become permanent.
Some scientists have begun to cast doubt on official estimates of the rate of oil flow, saying the widely repeated figure of 5,000 barrels of oil per day dramatically understates the real amount.
The spill is threatening to eclipse the 1989 Exxon Valdez leak off Alaska as America’s worst environmental disaster.
Underwater efforts to cap oil leak
Initially, BP tried to lower a 125-tonne, 18-metre (40 feet) high container dome over the main leak on the sea floor. However, this failed when gas leaking from the pipe mixed with water to form hydrates, ice-like crystals, that blocked up the steel canopy.
Instead, engineers have lowered a smaller device onto the site. Dubbed the Top hat, it will sit over the tear in the pipe and partially stop the leak. To prevent the build up of hydrates, methanol is pumped into the top hat to disperse the water and gas.
The top hat is 1.5m (5 feet) high and 1.2m in diameter. Two special side lines are used to pump methanol into the top hat to displace water and gas leaking from the broken oil pipe. This should prevent the build-up of hydrates. Once in place, oil can be pumped up to the surface.
BP plan to lower the original subsea containment dome over the top hat to provide a better seal over the leaking site and pump oil up to the surface. This time, it will be attached to a pipe that can pump warm water into the dome to prevent the build-up of hydrates.
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