Two American soldiers and least two Afghans have been killed in the latest suspected insider attack, which soon after led to a second fatal incident.
Afghan officials said the incident on Monday took place in Wardak, a flashpoint province for Taliban violence on the doorstep of Kabul, the capital.
"Today one or several individuals wearing an ANSF (Afghan National Security Force) uniform turned his or their weapon at a group of Afghan ... and coalition service members and killed several on both sides," a NATO spokesman told the AFP news agency.
Al Jazeera's correspondent Jennifer Glasse, reporting from Kabul, said that a man wearing an Afghan police uniform opened fire with a mounted machine gun.
Our correspondent said the shooting reportedly went on for 20 minutes and that the incident would be the second insider attack in Afghanistan within a week.
Sediq Sediqqi, interior ministry spokesman, confirmed the location as Jalrez in Wardak and said a delegation had been sent from Kabul to investigate.
The incidents comes after the end of a two-week ultimatum from Hamid Karzai for Washington to remove a US-operated elite force from the troubled province because the Afghan president said they were fuelling "insecurity and instability" there.
In separate incident on Monday, Afghan officials said that US troops shot and killed two civilians as their truck was approaching a US convoy.
Sediqi said that the victims in the incident were employees of a company that repairs police vehicles.
US forces' spokesman Jamie Graybeal said the vehicle failed to heed instructions to stop as it came close to the convoy outside of Kabul, and that the soldiers took "appropriate measures to protect themselves".
He confirmed that two individuals were killed and said an assessment was under way.
A video camera caught on camera an American major as he reacted angrily on the news of the killing.
Al Jazeera's Glasse said the US army officer's reaction "shows the kind of pressure and stress that US and NATO forces are under when dealing with civilians".
"Civilians casualties have been a difficult point between NATO and the Afghan government," Glasse said.