A huge force of US Marines leading NATO and Afghan soldiers is expected to launch an offensive in Helmand province within days.

The operation is said by commanders to be the largest assault against Taliban-led militants since the war began.

Operation Mushtarak (‘Together’) is intended to clear insurgents from their stronghold and ‘send a strong signal that the Afghan government is expanding its security control,’ said US General Stanley McChrystal, who leads 113,000 US and NATO forces.

The operation is to be centred on the Marjah plain in the central Helmand River valley, home to around 80,000 people.

As part of his counter-insurgency strategy emphasising development and governance, General McChrystal said the Marjah operation was not about killing Taliban fighters but eradicating the militant threat.

Whether fighters leave the region or rejoin society - as President Hamid Karzai's reconciliation programme encourages them to do - the aim is to establish Afghan civilian governance, he said.

He said it is an Afghan-led operation, with military supporting a civilian plan.

Preparatory operations around Marjah, south of Lashkar Gah, have been going on for weeks, with leaflets dropped on the area from NATO helicopters warning residents of the assault to come.

The head of the provincial refugees and repatriation department said authorities were preparing to receive up to 10,000 people, as they flee the area. About 2,000 have already left Marjah.

Marjah is a major base for growing poppies, the raw material of opium and heroin, which helps fund the insurgency.

Officials say farmers are coerced by militants into growing poppies rather than other crops.