Workers are trying a new tactic to stem the flow of oil gushing from a well on the floor of the Gulf of Mexico involving a pipe 1.6 kilometres long.

Engineers piloting deep-sea robots will use the pipe, which has been fitted with a rubber stopper and housed in another piece of equipment called the framework, to try to plug the massive leak and send the crude through the pipe to the surface.

The idea is that once that equipment is in place, crews will be able to funnel the oil to a tanker floating above the leak.

Engineers hit a snag when they tried to connect the two pieces of equipment 1.6 kilometres below the water's surface.

Doug Suttles, BP's chief operating officer, said the framework was lowered and then brought back to the surface Friday night to readjust it. He said the company was working again to insert it and expects to be bringing oil up to the surface by Saturday night.

"The frame shifted, so they were unable to make that connection," said Suttles, who believes the adjustments will make the device work.

By one estimate, the pipe, which has been leaking for three weeks, is spewing 800,000 litres of oil a day, creating an environmental nightmare.

A containment dome, called a "top hat," was lowered to the seabed 1,500 metres below the water's surface on Tuesday night and might be used as a backup to the funnelling, Suttles said.

The top hat's predecessor, a 12-metre-tall, 90-tonne box, failed to contain the oil leak after a slushy mixture of gas and water clogged the opening in its roof.

Underwater chemicals OK'd

The U.S. government has approved the use of chemicals to try to disperse the oil far beneath the surface.

"This was not a decision that was made lightly," U.S. Coast Guard Rear Admiral Mary Landry said Friday. She said approval was granted only after a team of experts analyzed three tests in which underwater dispersants were used.

So far more than 1.95 million litres of dispersants, mostly a product called Corexit 9500 previously approved by the Environmental Protection Agency for use on the sea surface only, have been dropped over the spill or shot undersea.

Corexit 9500 is identified as a "moderate" human health hazard that can cause eye, skin or respiratory irritation with prolonged exposure, according to safety data documents.

Louisiana Health and Hospitals Secretary Alan Levine said federal regulators have dismissed state worries about the chemicals.

"Our concerns about the use of these dispersants underwater is based on the fact that there is virtually no science that supports the use of those chemicals," Levine said.

"We're trading off what we know is going to be environmental damage on the surface for environmental damage of a level we don't know that is going to be under the surface."

With files from The Associated Press