KANDAHAR, Afghanistan (Reuters) – U.S. troops shot four Afghan policemen dead and wounded another after mistaking them for militants during an operation in southern Afghanistan, a senior local official said on Tuesday.

Separately, on Tuesday three Afghans working for the American security firm USPI were hurt — and one was possibly killed — by a roadside bomb in Kandahar, provincial governor, Assadullah Khalid said.

He said there were no foreigners on board the vehicle, but had no further details about the attack outside Kandahar city.

The U.S. military in Kabul said it had no information about the killing of policemen which, if confirmed, would be the second incident of its kind this month.

Khalid said Monday night”s shootings in Maiwand district came after police fired into the air from their checkpoint.

&#34The U.S. soldiers were on a hunt for Taliban on the basis of a tip-off when the shot was fired in the sky and the soldiers thought it was the enemy who fired it,&#34 Khalid said.

&#34They fired on them and, after overrunning the checkpoint, found that they had killed four police and wounded another by mistake,&#34 he told reporters.

Earlier this month U.S. troops mistook some policemen for militants during a hunt in the adjacent province of Helmand, again killing four and wounding another.

The U.S. military, which leads some 20,000 foreign troops hunting militants in Afghanistan, confirmed that incident.

Nearly 11,00 people — most of them militants, but including more than 50 U.S. soldiers — have been killed in Afghanistan this year, the bloodiest period since 2001.

U.S.-led forces are blamed by many in the country for killing several thousand Afghans by mistake, mostly in airstrikes since the offensive to oust the Taliban was launched in October 2001.