Image caption BP says it shares public frustration at its inability to plug the leaking well

The US government has toughened its rhetoric towards BP as frustration builds about the company's inability to plug a leaking Gulf of Mexico oil well.

Interior Secretary Ken Salazar pledged to "keep our boot on their neck until the job gets done".

BP Plc would be accountable, "civilly and in whatever way is necessary", he said on a tour of the Gulf.

Earlier on Monday, BP pledged $500m (£346m) to study the spill's impact on the environment.

The company says it will be Wednesday before it tries a so-called "top kill" bid to plug the leak - firing heavy mud and cement down into the blown out well 5,000ft (1,500m) below the surface of the Gulf.

A BP official said the bid - which comes more than a month after the Deepwater Horizon oil rig exploded, killing 11 - stood a 60-70% chance of success.

BP Chief Operating Officer Doug Suttles said if the "top kill" failed there was a chance the flow of oil could temporarily be increased.

He said if that happened BP would immediately install a new containment device to go over the old riser.

Familiar mantra

A delegation of White House officials and senators - led by Mr Salazar and Homeland Security Secretary Janet Napolitano - flew over the Louisiana coastline on Monday to assess the extent of the pollution.

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The BBC's Paul Adams in Washington says the officials both repeated a now-familiar mantra: BP is responsible for the damage and the administration will maintain pressure on the oil company until the crisis is over.

Ms Napolitano said: "We are going to stay on this and stay on BP until this gets done and it gets done the right way."

Meanwhile, the Republican governor of Louisiana, Bobby Jindal, is voicing frustration at aspects of the federal government's response.

It seems likely that the longer this environmental crisis drags on, without any convincing end in sight, the more vulnerable the administration will be to suggestions that it could and should be doing more, our correspondent says.

It is also fending off charges that offshore drilling permits and environmental waivers continue to be granted despite a moratorium announced by the president.

Mr Salazar said the pause was still in effect and would be until lessons had been learnt and additional safety measures put in place.

Also on Monday, BP pledged to fund a 10-year research effort aimed at studying the long-term effect of the spill on the shoreline and marine environments in the Gulf of Mexico, and the impact of the response efforts.

"This will be a key part of the process of restoration, and for improving the industry response capability for the future," BP chief executive Tony Hayward said in a statement.

'Missed deadlines'

In a round of appearances on US television news programmes on Monday morning, Mr Suttles said BP was aware that frustration with the company was growing over its inability to stop the leak.

The fact that it's taken this long is painful to everybody Doug Suttles, BP Chief Operations Officer In pictures: Spill's impact

"We are doing everything we can, everything I know," Mr Suttles told NBC's Today show.

"Everyone is frustrated," he added.

"I think the people of the region are frustrated. I know we are, I know the government is. The fact that it's taken this long is painful to everybody."

Meanwhile, the US government regulators who oversee offshore oil drilling have granted at least 19 environmental waivers for gulf drilling projects and 17 drilling permits since the oil leak began more than a month ago, the New York Times reported on Monday.

The new permits and waivers would appear to contradict President Barack Obama's declaration of a moratorium on new offshore drilling, issued soon after the rig explosion