US scientists say they have discovered enormous plumes of oil in the deep waters of the Gulf of Mexico, suggesting that the leak from the blown-out well operated by BP could be far worse than previously estimated.

They are quoted in the New York Times as saying one of the plumes is 16 kilometres long, nearly five-kilometres wide and 91 metres thick.

The scientists say the plumes are depleting the oxygen in the gulf, prompting fears that the process could eventually kill much of the nearby sea life.

The Interior Secretary Ken Salazar says the public has a right to know that the leak will be capped soon.

"That cancer has got be stopped and so we have been working relentlessly in... [assembling] the best group of scientists around the world, working with Secretary Chu and myself, that are providing an oversight to BP and making sure that everything that can be done that holds the well down is being done," he said.

But BP earlier said it was confident it would soon be able to stop the flow of oil.

The US government has now approved the use of dispersants under water near the source of the leak and BP says they are starting to work.

Engineers are also using underwater robots in an attempt to reinsert a tube surrounded by a stopper into the broken pipe.

They hope this will help them siphon the oil to the surface.

Doug Suttles from BP says the operation is continuing and he is confident it will start working in the next few hours.

"We're in the process of setting up the riser insertion tube," he said.

"This is the method to contain the flow. This doesn't stop the flow but it contains the flow.

"We hope to have that tool inserted by some time late tonight (local time). It's back on the seabed."

The White House is calling for immediate clarification that BP will pay for the whole clean-up operation.

In a letter to BP, the interior secretary Ken Salazar and the homeland security secretary Janet Napolitano say the public has a right to know the company's true intentions.

The letter from the Obama administration effectively asks BP to make clear how much it will pay for the Deepwater Horizon disaster.

Last week the chair of BP's American operations said he believed that statutory $75 million cap on a company's liability for oil spills was irrelevant.

He added that BP will pay for all clean-up costs and legitimate damages claims.

Meanwhile, BP's chief executive appears to have dismissed the massive oil slick caused by the gusher as being blown out of proportion.

In an interview in Britain's Guardian newspaper, Tony Hayward described the Gulf of Mexico is "a very big ocean", saying the volume of oil and dispersant going into it is "tiny" compared to the total volume of water.

- ABC/AAP/BBC