The wetlands off the Louisiana coast sustain hundreds of wildlife species and a big seafood and fishing industry.

Governor Bobby Jindal has declared a state of emergency and asked for federal funds to deploy 6,000 National Guard soldiers to help with the clean-up.

The Coast Guard said it had sent investigators to confirm whether crude oil had begun to wash up on parts of the Louisiana shoreline.

David Kennedy of the National Oceanic and Atmospheric Administration described the oil spill as a “very, very big thing”.

“No additional drilling has been authorised and none will until we find out what happened,” White House adviser David Axelrod told ABC television.

Last month President Barack Obama eased a moratorium on new offshore drilling.

Up to 5,000 barrels of oil a day are thought to be spilling into the water after last week’s rig explosion.

The slick has begun to reach the Louisiana shore, and the US Navy has been sent to help avert an economic and environmental disaster.

‘Mind-boggling’

Mr Axelrod announced the ban on drilling in new areas in an interview with ABC’s Good Morning America programme on Friday.

He also defended the administration’s response to the 20 April explosion that destroyed the BP-operated Deepwater Horizon rig, saying “we had the Coast Guard in almost immediately”.

The PM, who snap polls suggested was seen to come third in the debate, said the TV clashes were over and decision time for voters was beginning.

Tory leader David Cameron said the election was “far from won”.

Lib Dem leader Nick Clegg said the election was now a “two-horse race” between his party and the Tories.

In other election developments on Friday:

* The Conservatives send out a promises “contract” to voters in target seats
* Former PM Tony Blair says Labour has “every chance of succeeding” in the election on 6 May
* Nick Clegg says the election is now a two-horse race between the Lib Dems and Tories
* Audience figures suggest 8.4m on average watched the last TV debate on the BBC
* The SNP and Plaid Cymru outline what they will do if there is a hung parliament
* Labour launches a poster campaign based around their child tax credit pledge – as a car crashes into a bus shelter nearby
* UKIP says it would axe a dozen quangos to save billions of pounds
* Thousands of postal votes are to be sent out again in the Vale of Glamorgan, a key marginal seat, after an error invalidated some forms.

Snap polls suggested Mr Cameron was seen to do best in the final live debate – watched by 8.4m people on average – peaking at 9.1m at 2100 BST.