BP was trying late Thursday night to secure a lid on the broken pipe that has gushed more oil into the Gulf of Mexico than any spill in U.S. history.

A live video stream depicted robotic submarines positioning a funnel-shaped cap over the pipe, 1.5 kilometres below the sea.

"We'll have to see when we get the containment cap on it just how effective it is," said Coast Guard Adm. Thad Allen, a spokesperson for the U.S. government.

If successful, oil caught by the funnel would flow up a tube to an oil tanker on the surface. Engineers said some oil would still escape from the pipe, which was sawed off earlier on Thursday. But a rubber seal on the cap would minimize the spillage.

"It's an important milestone, and in some sense, it's just the beginning," BP CEO Tony Hayward said.

Hayward's comments came as BP began a carefully crafted public apology Thursday, taking to the American airwaves to say "I'm sorry."

"The Gulf spill is a tragedy that never should have happened," Hayward says in the spots. "BP has taken full responsibility for cleaning up the spill in the Gulf"

"We will honour all legitimate claims. And our cleanup efforts will not come at any cost to taxpayers," he says.

The company delivered the same message in full-page ads in newspapers such as The New York Times, the Wall Street Journal, USA Today and The Washington Post.

Since the oil spill, BP has lost 15 per cent of its market value -- about $21.1 billion.

Meanwhile, the enormous tentacles of spilled oil are spreading closer to new areas of the U.S. coastline. Oil was reportedly floating 6 kilometres off the Florida coast Thursday evening and could be washing up on local shores as early as this weekend.

"I think we've already lost a season," said Beth Schachner, with Pensacola Beach Properties. "We're trying really, really hard. We continually ask our tenants to hang in there with us."

The spill has also become a growing political problem for U.S. President Barack Obama, who will return to the Gulf Coast on Friday.

"I am furious at this entire situation, because this is an example of where somebody didn't think through the consequences of their actions," Obama said on CNN's "Larry King Live."

The Associated Press also reported Thursday that the Obama administration is blocking all new offshore drilling in the Gulf of Mexico.

An email from the Gulf Coast office of the Minerals Management Service, obtained by AP, says that "until further notice" no new drilling is being allowed in the Gulf, no matter the water depth.

The announcement comes a day after the agency, which oversees offshore drilling, granted a new drilling permit for a site about 80 kilometres off the Louisiana coast, at the relatively shallow depth of 35 metres below the ocean surface.

Environmental groups have accused Washington of misleading the public by allowing work to resume in waters up to 152 metres deep while maintaining a moratorium on deepwater drilling.

The U.S. Coast Guard said earlier in the day that underwater robots have successfully sliced through a pipe atop the blown-out well in the Gulf. But the cut was described as irregular, which makes placing a cap over the broken well more challenging.

If the strategy fails -- like every other attempt so far has done -- the best hope is probably a relief well, which is at least two months from completion.

The edge of the slick has already washed up on the Mississippi and Alabama shores, after fouling 200 kilometres of Louisiana coastline. Now residents of Florida's panhandle are bracing for the worst.

"It's inevitable that we will see it on the beaches," said Keith Wilkins, deputy chief of neighbourhood and community services for Florida's Escambia County.

The slick will threaten a delicate network of islands, bays and sugar-white sand beaches that are a major tourist destination jokingly dubbed "the Redneck Riviera."

Emergency workers are rushing to protect the beaches along the eastern Gulf Coast as best they can, by linking the last in a miles-long chain of booms.

Two cutters have been mobilized to help, in Mobile Bay, Ala., and one off Pensacola. The boats will help skim oil and add more boom to collect it.

It's now been 45 days since the Deepwater Horizon rig exploded, killing 11 workers. The rig was being operated for BP, the largest oil and gas producer in the Gulf.

Over the past six weeks, the well has leaked anywhere from 21 million to 45 million gallons by the government's estimate. That's made the spill the biggest in U.S. history, releasing more than three times the amount of crude in the Exxon Valdez disaster.

With files from The Associated Press