General David McKiernan was replaced after less than a year in the job. The new commander will be Lieutenant General Stanley McChrystal, also an Army officer but with experience as a leader of special forces.

Gen McKiernan issued a short statement saying his time in Afghanistan made him proud to be an American soldier.

Gen McKiernan's exit comes as more than 21,000 additional US forces begin to arrive in Afghanistan, dispatched by President Obama to confront the Taleban more forcefully this spring and summer. Despite seven years of effort by the US and allies, Afghanistan remains a battleground with an unstable government, flourishing opium trade and suicide attacks by supporters of Al-Qaeda.

Monday's announcement came a week after Afghan civilians were killed during a battle between militants and US forces.

Afghan officials say up to 147 people may have died in the battle in Farah, though the US says that number is exaggerated.

The US on Saturday blamed Taleban militants for causing the deaths by using villagers as human shields in hopes they would be killed. A preliminary US report did not say how many people died in the battle.

Mr Gates said McChrystal, now a senior administrator with the Joint Chiefs of Staff, would be nominated for the top job in Afghanistan and that Lt Gen David M. Rodriguez would become McChrystal's deputy. The defence secretary urged the Senate to confirm the two quickly.

President Obama approved 17,000 additional combat forces for Afghanistan this year, plus 4,000 trainers and other noncombat troops. By year's end, the United States will have more than 68,000 troops in the sprawling country - about double the total at the end of George W. Bush's presidency but still far fewer than the 130,000 still in Iraq.

Gen McKiernan and other US commanders have said resources they need in Afghanistan are tied up in Iraq. -- AP