NEW ORLEANS, Louisiana (AFP) – BP said Thursday its bid to cap the Gulf of Mexico oil leak with cement was on track as submarines toiled round the clock while fumes forced boats involved in the clean-up back to port.

"The job has been proceeding according to plan," BP Chief Operating Officer Doug Suttles said after the start of the maneuver dubbed a "top kill" began amid huge pressure to finally staunch the five-week-old spill.

US officials meanwhile said all 125 commercial fishing boats helping to clean up the oil off Louisiana's Breton Sound were recalled after four workers reported health problems.

The crew members aboard three separate vessels working in the area "reported experiencing nausea, dizziness, headaches and chest pains", raising questions over the toxicity of chemical dispersants used by BP to break up the slick.

After several previous failed attempts to cap the oil, BP boss Tony Hayward has already downplayed hopes for success with the 'top kill,' cautioning the procedure has never been tried before at such depth and against such pressure.

He warned it was expected to take two days to complete the difficult operation to inject heavy drilling fluids into the oil flow and then seal it with cement.

"We just need to take the next 24 hours and see what the results are," Suttles told reporters after being asked BP's level of optimism after the process was begun.

The work being carried out by remote-controlled robotic submarines a mile (1,600 meters) below the surface aims to counterbalance the oil flow with the injected fluids, drowning the leak long enough to dump cement on top and permanently seal it.

White House deputy spokesman Bill Burton told reporters on Air Force One that President Barack Obama was being updated on the progress.

"I would say that his level of frustration is very high and that every moment that that hole is not plugged the president has a deep level of concern," he said.

Obama was Thursday to take questions at a press conference over the government's handling of the spill ahead of his second visit to the region on Friday.

The Deepwater Horizon rig, 50 miles (80 kilometers) off the Louisiana coast, exploded on April 20, killing 11 workers.

Its fractured pipe has been spouting oil for 36 days, creating a massive slick washing up along the coast and threatening endangered birds, animals and plants.

Oil has now soiled more than 100 miles of Louisiana coastline, state Governor Bobby Jindal said Wednesday, more than doubling the previous estimate.

A tour of coastal areas left the president of the local Plaquemines parish aghast at the devastation -- and what he described as an incompetent response.

"The same oil that's been out there two weeks ago is still out there. And nothing is being done," Billy Nungesser told CNN.

BP has previously only managed to siphon up some oil via a tube inserted into the pipe last week and Hayward has put the chances of the "top kill" success at 60 to 70 percent.

Officials are also readying back-up options but some, including the drilling of relief wells to divert the flow and allow the original well to be capped, could take several months.

Obama is also expected to announce tough new offshore oil regulations Thursday after receiving an Interior Department report into what has become one of the worst oil spills in US history.

In a hint of what might be in the report, Interior Secretary Ken Salazar told lawmakers that "there are significant enhancements that can be made with respect to the safety of outer continental shelf oil and gas development."

The response by BP, Obama and the government all got bad grades from Americans in a new USA TODAY/Gallup Poll.

Nearly three-fourths of those surveyed Monday and Tuesday said BP was doing a "poor" or "very poor" job. Sixty percent said the same about the federal government, while 53 percent slapped Obama with a poor rating on the crisis.

Thanks to a webcam which BP has placed close to the leak, the whole procedure is being aired live on US television, with news channels streaming footage of the oil gushing from the broken well pipe.

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