The Taliban claimed responsibility for a blast on Monday that the US military said killed four troops.

The Taliban said nine Americans were killed and US forces were helpless in the face of such attacks.

The soldiers died on Monday in Uruzgan province, one of the hotbeds of an insurgency launched by the Taliban after they were removed from government in a US-led operation four years ago.

An exchange of small arm and rocket-propelled grenade fire erupted after the explosion, and the coalition called in helicopters and planes to help the US and Afghan forces on the ground, a coalition statement said.

"Taliban attacks will further increase with a decrease in the winter cold," a former Taliban governor of Kandahar province, Mullah Mohammad Hassan Rahmani, told the agency by satellite telephone from an undisclosed location.

Fighting in Afghanistan traditionally eases off during the winter when mountain passes get snowed under.

But violence has surged in recent months, including 15 suicide blasts since November, as Nato members led by Britain, Canada and the Netherlands prepare to expand their peacekeeping mission.