KABUL - A total of three US soldiers were killed by a man in Afghan military uniform on Friday morning, the US Forces-Afghanistan said in a statement. ?

"Three US Forces-Afghanistan (USFOR-A) service members died following an attack by an individual wearing an Afghan uniform in southwest Afghanistan today," said the statement.

The brief statement did not disclose the name of the killed soldiers, only saying "it is USFOR-A policy to defer casualty identification procedures to the US Department of Defense."

Meantime, an Afghan official said that the incident took place in a joint Afghan-coalition checkpost in Bostanzoi area in Sangin district of the southern Helmand province at around 2 am local time Friday.

According to coalition officials, southern Helmand belongs to coalition's Regional Command Southwest.

The Friday's attack is the latest in the so-called "green-on- blue" attacks when members of Afghan army or police turned their weapons against coalition forces. ?

Earlier Friday, administrative chief of Sangin district, Mohammad Sharif, told Xinhua that a police officer shot dead four Afghan Special Force members. However, later he said that the police officer shot and killed three cops with the coalition forces.

"The attacker escaped the area shortly after the shooting and a joint investigation was launched into the incident," he said.

Up to 20 such attacks took place since January 1 this year, leaving more than two dozen US and NATO soldiers dead and several others injured.

Two Afghan army soldiers opened fire on coalition soldiers in Shwak district of eastern Paktia province, killing a US soldier and injuring four other US soldiers on Tuesday.