It will be at least Wednesday before BP will try using heavy mud and cement to plug a massive oil leak in the Gulf of Mexico, a company spokesman said Monday.

The manoeuvre, called a top kill, represents the best hope of stopping the oil after several failed attempts.

BP initially said it would try Tuesday, but John Curry said more time is needed to get equipment in place and test it.

"Our goal, of course, is to succeed," Curry said. "We want this as much as anyone and our best chance of success is looking like Wednesday morning."

Meanwhile, Doug Suttles, chief operating officer at BP PLC, went on all three U.S. network morning talk shows to say he knows people are frustrated by its failure to plug the well that has been gushing for more than a month and is now spreading damage through Louisiana's wetlands.

"We are doing everything we can, everything I know," he said on NBC's Today show.

Hundreds of millions of litres of oil have already spewed from the well that blew out after a drilling rig exploded April 20 off the Louisiana coast.

'We are going to stay on this'

Several officials from President Barack Obama's administration led a delegation of U.S. senators who surveyed the affected areas from the air Monday and then held a news conference to emphasize that the cleanup is BP's responsibility.

"We are going to stay on this and stay on BP until this gets done and it gets done the right way," said Homeland Security Secretary Janet Napolitano.

BP said Monday its costs for the spill had grown to about $760 million US, including containment efforts, drilling a relief well to stop the leak permanently, grants to Gulf states for their response costs, and payment of damage claims. BP said it's too early to calculate other potential costs and liabilities.

At least 22.7 million litres of crude have spewed into the Gulf of Mexico, according to a U.S. Coast Guard and BP estimate.

A 1.6-kilometre tube operating for about a week sucked off more than 216,000 litres on Sunday, but it began sucking up oil at a slower rate over the weekend, and even at its best it wasn't capturing all of what is leaking.

The spill's impact on shore now stretches across 241 kilometres, from Dauphin Island, Ala., to Grand Isle, La.

With files from The Associated Press