4 US soldiers killed in Afghan mine blast

"Four US troops were killed in action but we have no indication that this was a new mine or an IED (improvised explosive device)," US military spokeswoman Cindy Moore told AFP.

She said the bodies of the US troops had already been evacuated from the area and an investigation was ongoing to determine the cause of the explosion, which occurred 40km southwest of the capital Kabul.

"The four soldiers were killed when their vehicle hit a mine, as they were moving in a three-vehicle convoy on a joint mission with the Afghan National Army," Moore said.

The patrol was surveying for a weapons range and they had been in the area a week and a half, she added.

The blast, which Afghan intelligence sources said occurred in Mohammed Agha district in the Shikar Qala area of Logar could have been the result of unexploded ordnance as Afghanistan is littered with old mines after a quarter century of war.

Saturday's explosion brings to seven the number of US soldiers killed so far this year in Afghanistan.

A US soldier was killed in the western region of Herat in a mine blast earlier this month.

The US has more than 20,000 troops in Afghanistan, most of whom are battling insurgents in the south and southeast of the country where three years after the fall of the Taliban loyalists of the regime continue to wage an insurgency.

After a lull in violence during Afghanistan's bitterest winter for more a decade, attacks have begun to pick up as the weather warms.

A US soldier was killed and four others wounded in the western city of Shindand on March 16 in a blast, which killed eight Afghan civilians and wounded three others.