Thousands of US-led troops backed by helicopters yesterday stormed an Islamist stronghold in southern Afghanistan in Nato's biggest operation since the Taliban regime's overthrow in late 2001.

US Marines led the charge on Marjah, a town of 80,000 in the central Helmand River valley controlled for years by militants and drug traffickers, in the first major test of President Barack Obama's new surge policy.

US, British and Afghan soldiers dropped into Marjah from helicopters before dawn, immediately coming under fire and claiming their first Taliban victims within hours, Afghan army and Marines officers said.

Operation Mushtarak ('together' in Dari), as the assault involving 15,000 troops is known, aims to clear the area of Taliban and re-establish Afghan sovereignty and civil services, Helmand Governor Mohammad Gulab Mangal said.

British forces suffered their first casualty of the operation when a soldier was killed in an explosion while on a vehicle patrol in Helmand province's Nad-e-Ali area, Britain's Defence Ministry said in London.

Nato's International Security Assistance Force (ISAF) said five foreign soldiers died yesterday in the south of Afghanistan, three of them US troops, but did not say if they had been involved in the Marjah attack.

At least 20 Taliban fighters were killed in the first hours of the assault, said General Sher Mohammad Zazai, commander of the operation's Afghan troops.

"So far, we have killed 20 armed opposition fighters. Eleven others have been detained," he said, adding they were killed in separate engagements.

Afghan Defence Minister Abdul Raheem Wardak said the operation was making painstaking progress because the area had been laced with deadly improvised explosive devices (IEDs), which can be almost impossible to detect.

"The area has been heavily mined, that's why we are moving so slowly," Wardak told reporters in Kabul.

Nato commanders were nevertheless satisfied with the operation's progress thus far, according to a senior British military spokesman.

The commanders were "very pleased with how it has gone," Major General Gordon Messenger told a briefing in London.

"The key objective has been secured," he said, explaining that the main aims for British troops were to secure the population centres and installations such as police stations in the the Chah-e Anjir Triangle northeast of Marjah.

There had been some "sporadic fighting", but the Taliban appeared to be "confused and disjointed" and "have not been able to put up a coherent response," Messenger said.

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