KANDAHAR: An air attack has killed at least 11 Afghan police officers in the volatile and opium-rich southern province of Helmand, officials said on Monday, in the latest apparent case of “friendly fire”.

An interior ministry spokesman said the strike on Sunday was carried out by Nato forces but another ministry official could not confirm who was responsible.

Nato said in a statement it conducted no air strikes in Helmand on Sunday.

Afghanistan's own air force also has the capability to carry out aerial bombardments, but both the force and the defence ministry declined to comment.

“Eleven counter-narcotics police were killed and four others were wounded in an air strike carried out by international forces in Garmsir district of Helmand province,” deputy interior ministry spokesman Najib Danish told AFP.

A spokesman for the counter-narcotics police at the interior ministry confirmed the toll but could not say who carried out the attack.

Helmand, the heartland of the global opium trade, is one of the biggest flash-points in the 14-year Taliban insurgency that erupted after a US-led invasion brought down their regime in 2001.

US-led Nato forces ended their combat mission in Afghanistan last December, leaving local forces to battle militants alone on the ground.

A 13,000-strong US-dominated residual force remains for training and counter-terrorism operations. But there has been an escalation in air strikes by Nato forces in recent months despite the draw-down.

Foreign coalition forces carried out 106 air strikes in June, a sharp jump compared to 41 the previous month, according to Nato statistics.

That figure is still significantly lower than previous years.

But NATO forces have stepped up air attacks in Helmand in recent months after the Taliban advanced on some districts.

From August 22-29 US forces conducted 18 air attacks in and around Musa Qala district to destroy Taliban positions, according to a Nato spokesman.

The strategic district recently fell to the Taliban but was subsequently recaptured.

While “friendly fire” incidents involving foreign coalition forces are a deeply contentious issue in Afghanistan, UN statistics show that the Taliban are responsible for most deaths.

Foreign forces are themselves also known sometimes to fall victim to “friendly fire” incidents.

A Nato airstrike in June last year killed five US troops and an Afghan soldier in an accident during clashes with insurgents.

The incident could have been avoided if American forces had communicated properly and understood their aircraft's capabilities, military investigators later said.

US President Barack Obama has backpedalled on plans to shrink the US force in Afghanistan this year by nearly half, agreeing to keep the current level of 9,800 US troops until the end of 2015.