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The operation was aimed at putting pressure on insurgents "and to show our commitment to the Afghan people that when we come in we are going to stay long enough to set up their own institutions," Captain Pelletier said.

He said the US military was prepared for casualties, but stressed that "it is absolutely essential that no civilians be harmed".

"We do not want people of Helmand province to see us as an enemy, we want to protect them from the enemy," he said.

The Marines hope by appearing suddenly and in overwhelming numbers, they can capture some of the Taleban's firmest strongholds with little resistance.

"Towns that were the Taleban heartland will fall. They will fall quickly. And hopefully they will fall without a shot. That's our intent," Brigadier General Nicholson said.

However, the greater challenge will be holding and stabilising such gains against Taleban re-infiltration and convincing a highly sceptical local population that Western forces will offer long-term security and improvements to their lives. The developing symbiotic relationship between the Taleban and Helmand’s drugs mafia will further complicate that process. The province is the largest producer of opium in the world, the raw base for 90 per cent of the heroin used by British drug addicts. Drugs money has become a major source of Taleban funding and hundreds of thousands of local people are involved in the production and harvesting of opium poppy.

The insurgents have proved adept at reinfiltrating behind Western forces using the local civilian populace as cover. It is a problem that has beset the 8,000 British troops who have been thinly spread across Helmand, the country’s largest province and roughly akin to twice the size of Wales, since they were deployed to the region in 2006.

Though Britain has doubled its troop presence since an initial deployment in 2006, they have been too few in number to do more than take and hold a few key islands of territory in the province.

Nato internal documents seen by the Times concede that 5 of the 13 districts of the province currently have no Afghan government presence at all. Ahead of the August elections Western forces are likely to attempt to cut the Taleban’s supply lines southward, across the border to safe havens in Pakistan, whilst deluging the highly populated central parts of the province with Western troops.

The 10,000 Marines in Helmand Province, 8,500 of whom arrived in the last two months, form the biggest wave of an escalation ordered by President Obama.

He has declared the Taleban insurgency in Afghanistan and neighbouring Pakistan to be America's main foreign threat. Insurgent attacks in Afghanistan are at their highest since the militants were toppled in 2001.

Under President Obama the US force in Afghanistan is more than doubling this year, from 32,000 at the start of 2009 to an anticipated 68,000 troops by the end of the year, many of them diverted from Iraq. Other Western countries have about 33,000 troops in Afghanistan.

Addressing Marine commanders days before the assault, Dutch Major-General Mart de Kruif compared it to the D-Day invasion that changed the course of World War Two.

"We have people out there who do not realise that progress is about to come to them," he said. "We have enemies out there who do not yet realise that they are going to lose."

The governor of Helmand province, Gulab Mangal predicted the operation would be "very effective".

"The security forces will build bases to provide security for the local people so that they can carry out every activity with this favourable background and take their lives forward in peace," the governor said in a Pentagon news release.

Local phone masts in the area affected by the operation appeared to have been switched off this morning, though it was not clear whether this was due to US or Taleban action.

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