The assault appeared to the be the latest in a string of insider attacks that have threatened to sever the partnership between international troops and the Afghan forces they are trying to train to take over responsibility for the country's security. There have also been cases of insurgents donning Afghan uniforms in assaults.
A statement from NATO gave no further details, saying the shooting is still under investigation.
Afghan officials said there was an attack in Helmand province's Nahri Sarraj district but also could not confirm any details.
"We know that there are casualties," said Ismail Hotak, the director of the provincial office that coordinates with the international forces.
Both the British and American militaries have large contingents in Helmand.
At least 53 international troops have been killed in attacks by Afghan soldiers or police this year, and a number of other assaults are still under investigation, the international alliance has said.
The surge in insider attacks is throwing doubt on the capability of the Afghan security forces to take over from international troops ahead of a planned handover to the Afghans in 2014. It has further undermined public support for the 11-year war in NATO countries.
The attacks have not been limited to members of the NATO-led international coalition. More than 50 Afghan members of the government's security forces also have died this in attacks by their own colleagues.