Sarkozy called it "unacceptable" that Afghan soldiers would fire at French troops.
Afghan officials said another 17 French soldiers were wounded in the incident, which took place in eastern Kapisa Province.
France has 3,600 troops serving with the NATO-led International Security Assistance Force (ISAF). More than 80 French soldiers have been killed in Afghanistan since 2001.
Incidents of NATO soldiers being killed by Afghan army troops have increased in recent months, undermining trust in the leadup to the withdrawal of foreign combat troops in 2014.
ISAF initially announced the January 20 incident in a statement, saying the perpetrator was an Afghan National Army soldier.
ISAF said the suspected shooter had been detained.
Further details were not imediately available.
compiled from agency reports