The attack in the flashpoint eastern province of Wardak by an individual in an Afghan army uniform also killed several Afghan soldiers and wounded a number of others, military officials said.
The insider attack in Wardak came after the end of a deadline Karzai set for the departure of US elite special forces from the strategically important province, accusing them of fuelling instability. More than 60 foreign soldiers were killed in 2012 in insider attacks that have bred mistrust and threatened to derail the training process.
The most recent came on Friday in the restive eastern province of Kapisa, where a Nato civilian contractor was killed by three people wearing Afghan security force uniforms.
"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 AFP.
The spokesman did not give details but a government spokesman in Wardak told AFP the incident had happened there and authorities were investigating.
Interior ministry spokesman Sediq Sediqqi confirmed the location as Jalrez in Wardak and said a delegation had been sent from Kabul to investigate.