Helicopter-borne US Marines and Afghan troops swooped down on the Taliban-held town of Marjah before dawn on Saturday, launching a long-expected attack to re-establish government control and undermine support for the militants in their southern heartland.

The attack on Marjah climaxed the biggest joint Afghan-international offensive of the war and is the largest combat operation since President Barack Obama ordered 30,000 US reinforcements here last December to turn the tide of the war.