Three American soldiers were killed on Friday when an Afghan police officer in Sangin district of Helmand province turned his weapon on them.

Two Afghan officials told AFP the soldiers were killed by an Afghan police officer who had invited them for a meal at his checkpost in the restive southern province.

“The police checkpost commander invited four foreign special forces soldiers to a (Ramadan) breakfast at 2:30 am in Sangin district,” a senior security officer in the province said, requesting anonymity.

“He later opened fire on the special forces soldiers, killing three and wounding another, and he managed to run away.”

The Sangin district chief, Mohammad Sharif, earlier told AFP that four foreign soldiers had been killed by the checkpost commander after he invited them to a meal.

An increasing number of Afghan soldiers and police have turned their weapons against NATO invaders.

On Tuesday, an American soldier was killed in the east when two men in Afghan army men opened fire, and on Thursday an Afghan soldier was killed after turning his weapon on NATO troops, also in the east.

The latest deaths take the green-on-blue toll this year to around 33, in some 23 such incidents, according to an AFP tally. 

NATO’s International Security Assistance Force (ISAF) has said the deaths sapped spirits among its troops.

“Although the incidents are small in number we are aware of the gravity they have as an effect on morale,” former ISAF spokesman Brigadier General Carsten Jacobson said.

Six civilians were killed and five others injured during a roadside bombing in southern Helmand province on Friday, an official said.

The Taliban on Friday rejected a UN report that held the militant movement responsible for 80 percent of civilian casualties during the first six months of the current year.

For the first time in five years, the report said, civilian casualties in Afghanistan dropped by 15 percent during the first half of the current year, compared to the same period in 2011.

Conflict-related violence caused 1,145 civilians to be killed and 1,954 injured during the period, the United Nations Assistance Mission in Afghanistan said in the 2012 Midyear Report on the Protection of Civilians in Armed Conflict.

"This reduction …reverses the trend in which civilian casualties had increased steadily over the previous five years," the report said, noting the devastating toll that the war continued to take on non-combatants.

The report said of the 3,099 civilian casualties, 925 were women or children, accounting for 30 percent of civilian casualties. The mission held militants responsible for 80 percent of the casualties.

However, the Taliban rejected the UN claim that they were behind 80 percent of civilian casualties. In a statement, the guerrillas said the report ignored the deaths and injuries caused by foreign troops.

Spurning the report as one-sided, the militants said the masses could not be duped with such reports. "It would be better for the UN, which has international credibility, to be realistic about the situation in Afghanistan.”

“We want the world body to be neutral and avoid moving in sync with our enemies in the propaganda war,'' the Taliban said, suggesting that they did their bit to spare civilians.