"A man wearing Afghan army uniform opened fire on French nationals, killing four and
injuring 16 others this morning in Tagab district of Kapisa province," the security official told the AFP news agency on condition of anonymity.
A spokesperson for Nato forces said earlier four troops from the international force in Afghanistan (Isaf) were killed and the killer had been arrested, but did not give the nationality of the troops.
French troops have sealed off the Tagab base, where the attack took place, and Afghan forces have been banned from entering.
- France is responsible for Isaf activity in Kapisa province;
- It was also responsible for Surobi district, east of Kabul, until President Hamid Karzai decreed that Afghan troops should take control there in November last year;
- Eighty-two French troops have died in Afghanistan since the fall of the Taliban in 2001 - 26 of them last year, all in Kapisa;
- There are about 3,800 French troops in Afghanistan, with 400 having pulled out since last October.
Last month a trainee Afghan soldier killed two French soldiers at Tagab.
Two French journalists, Hervé Ghesquière and Stéphane Taponier, were held hostage there for 18 months before being freed last year.
There have been several cases of foreign troops killed by attackers in Afghan uniform.
Six soldiers died on Thursday evening when their helicopter crashed in Helmand province.