A US official speaking on condition of anonymity said the soldiers were Americans.

Al Jazeera's Bernard Smith reporting from Kabul, the capital, said that the incident actually took place in the early hours of Thursday morning at a forward operating base in Kandahar.

The bases, which usually house about 100 soldiers, are often shared with Afghan national army soldiers, Smith said.

The civilian involved in the incident was believed to have been a local teacher, our correspondent reported.

"The local sources are telling us that just this teacher, snatched somehow, a weapon from an Afghan soldier, killing two  soldiers," Smith said.

In a statement, the NATO-led International Security Assistance Force (ISAF) said that one Afghan soldier and another man in civilian clothing had opened fired indiscriminately, killing two ISAF soldiers.

"ISAF are telling us that both the teacher and the Afghan national army soldier both opened fire. Both of those individuals were killed in the return fire," said Smith.

The killings on Thursday came after two senior US officers were shot dead inside the country's interior ministry on Saturday by a man who Afghan security officials said was a police intelligence officer.

That attack followed days of protests and clashes in Afghanistan over reports that copies of the Quran had been burnt at a US-run NATO base and prompted the military alliance to withdraw all its advisers from Afghan ministries.

It also raised questions about NATO's strategy of replacing large combat units with advisers as the alliance tries to wind down the war. 

According to US defence officials, about 70 members of the NATO-led ISAF force have died in insider attacks between May 2007 and January 2012.