KANDAHAR, Afghanistan: An Afghan police officer opened fire on four American soldiers he had invited for a meal Friday, killing three of them, Afghan officials said, in the third so-called green-on-blue assault in just four days.

The US military in Afghanistan confirmed that three US soldiers had been killed by "an individual in an Afghan uniform" in Sangin district of southern Helmand province, but gave no further details.

The deaths take the toll of those killed in green-on-blue attacks, in which Afghan forces turn their weapons against their NATO allies, to around 33 this year in some 23 such incidents, according to an AFP tally.

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

"Asadullah, 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."

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

Breakfast is eaten before sunrise during the holy month of Ramadan, when Muslims fast from then until the evening meal at dusk.

Taliban insurgents claimed responsibility for the attack, saying seven US special forces soldiers had been killed.

"Asadullah joined the mujahideen ranks after the killing," Taliban spokesman Qari Yousuf Ahmadi told AFP by telephone.

The Taliban regularly exaggerate attacks or claim credit for killing foreign soldiers even if they are not involved.

An increasing number of Afghan soldiers and police have turned their weapons against NATO colleagues helping them to fight a decade-long insurgency by the Taliban Islamists, who were ousted from power in a US-led invasion in 2001.

The attacks are raising questions about the ability of Afghan security forces to take over from NATO combat troops, the bulk of whom are scheduled to withdraw at the end of 2014.

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

Some of the attacks are claimed by the Taliban, who say they have infiltrated the ranks of Afghan security forces, but many are attributed to cultural differences and antagonism between local and US-led allied forces.

The White House said Friday the military impact of Afghan forces turning their guns on NATO trainers was "negligible".

"It is too early to say that this latest incident is part of a stepped-up effort on the insurgents' part," White House spokesman Jay Carney said.

"We know that they are trying to maximise media exposure of these events.

"But our military believes that the operational impact has been negligible. Nonetheless, these incidents do concern us. And as I said, our hearts go out to all the victims and their families."

Some 130,000 NATO troops in Afghanistan are training and working alongside Afghan soldiers and police as they take increasing responsibility for the anti-insurgency campaign.

While Western politicians keen to get their troops out of an unpopular war regularly talk up the ability of the Afghan army and police, the green-on-blue attacks resonate deeply with international forces.

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 earlier this year.

"Every single incident has an out-of-proportion effect on morale and that goes for coalition forces as it goes for Afghan national security forces."

Among measures being taken to prevent such attacks, Afghanistan's intelligence services are hiding agents among new recruits at army and police training schools to try to spot potential gunmen, NATO said.

ISAF has also taken several security measures in response to the shootings, including assigning "guardian angels" -- soldiers who watch over their comrades as they sleep.