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.
Some 15,000 US, British and Afghan soldiers stormed the Islamist stronghold in Nato’s biggest operation since overthrowing the Taliban regime in 2001.
Troops dropped into Marjah from helicopters before dawn Saturday, immediately coming under fire and claiming their first Taliban victims within hours, Afghan army and Marines officers said.
Operation Mushtarak ("together" in Dari) aims to clear the area of Taliban and re-establish Afghan sovereignty and civil services, Helmand Governor Mohammad Gulab Mangal said.
Senior officials and Nato commanders said they were satisfied with the operation's first day, with Mark Sedwill, Nato's senior civilian representative, saying it "appears to be positive."
"I can't yet say how long it will take for this military phase to get to the point where we can bring in the civilian support from the Afghan government, we hope that will happen quickly," Sedwill told reporters in Kabul.
Sedwill, a former British ambassador in Kabul, is slated to play a key role in the operation, whose principal aim is to establish civilian administration in one of the Taliban's last strongholds in Afghanistan.
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.
Nato's International Security Assistance Force (ISAF) said five foreign soldiers died on Saturday 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.
NATO commanders were "very pleased with how it has gone," senior British mililtary spokesman 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 Chah-e Anjir Triangle northeast of Marjah.
There had been some "sporadic fighting," but the Taliban appeared to be "confused and disjointed" and unable "to put up a coherent response," he said.
Mushtarak is the first major assault on a Taliban stronghold since Obama announced in December that he was sending an additional 30,000 soldiers to Afghanistan in 2010.
The US and Nato already have 113,000 troops in the country battling the insurgents. Nato has pledged another 10,000, bringing the total to more than 150,000 by August.
Mushtarak puts into practice the new US-led counter-insurgency strategy combining the military objective of eradicating the Taliban with the need to replace their brand of harsh control with civilian authority.
The battle for Marjah, an agricultural plain that is the source of most of the world's opium, is the first real test of the strategy, devised by US General Stanley McChrystal, commander of foreign forces in Afghanistan.
Afghan President Hamid Karzai warned the troops to do everything possible to avoid harming civilians, a sensitive issue among war-weary Afghans.
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