Meanwhile, Pakistan said it had deployed troops near its 1,600-mile border with Afghanistan to seal off a potential escape route for insurgents fleeing the US advance.

The Marine Expeditionary Brigade leading the operation represents a large number of the 21,000 additional troops that US President Barack Obama ordered to Afghanistan earlier this year amid the Taliban’s increasing domination in much of the country. The operation is the first major push in southern Afghanistan by the newly bolstered American force.

Shortly after the offensive began, a US military spokeswoman said insurgents had captured a US soldier in eastern Afghanistan on Tuesday. Capt Elizabeth Mathias said the soldier was not part of the offensive in Helmand, but gave no further details.

Helmand is one of the deadliest provinces in Afghanistan where Taliban fighters have practised sleek, hit-and-run guerrilla warfare against the British forces based there.

The British Defence Ministry said on Thursday that two British soldiers had been killed on Wednesday in a roadside bomb attack in Helmand and six foreign soldiers had been injured in the attack. The fatalities brought to 171 the number of British troops killed since the toppling of the Taliban Government in late 2001.

In recent weeks, British troops have been setting up “blocking positions”, apparently to help stop the flow of insurgents during the main military operation and to establish greater security before the presidential election in August.

Now, the Marines say their new mission, called Operation Khanjar, will include more troops and resources than ever before, as well as a commitment by the troops to live and patrol near population centres to ensure that residents are protected. More than 600 Afghan soldiers and police officers are also involved.

“What makes Operation Khanjar different from those that have occurred before is the massive size of the force introduced, the speed at which it will insert, and the fact that where we go we will stay, and where we stay, we will hold, build and work toward transition of all security responsibilities to Afghan forces,” Marine commander Brig Gen Larry Nicholson said in a statement.

The Marines will be pushing into areas where NATO and Afghan troops have not previously established a permanent presence.

The goal of the operation is to put pressure on the Taliban “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”, he said.

The 21,000 additional American troops that Obama authorised almost precisely matches the original number of additional troops that George W Bush sent to Iraq two years ago. It will bring the overall American deployment in Afghanistan to more than 60,000 troops. But Obama avoided calling it a surge.