NATO insists that murders of foreign troops by their Afghan colleagues are isolated incidents. But on one French base near Kabul, private doubts linger.

Nearly one in five NATO soldiers killed this year have died at the hands of their supposed allies -- including six Americans killed during riots against the burning of Korans at a US base last month.

On January 20, an Afghan soldier turned his weapon on his French trainers at Gwan base in the province of Kapisa, northeast of the capital Kabul, as they ended a sports session.

None of the French was armed. Four were killed and 15 wounded. And for many of their comrades, the wounds are still raw.

"Distrust would be a strong word," Colonel Didier Gros told AFP following a two-day Afghan-French operation in Usbeen valley near Kabul.

"There is a reality. Things have happened. We're not going to pretend as if nothing happened. Everyone has their own thoughts."

January's incident was the second in Kapisa in a month after two French soldiers were killed on December 29. As a result, France temporarily suspended training Afghan soldiers, who are due to take responsibility for national security by the end of 2014.

Normal training has since resumed but security has been tightened.

Afghan and French troops at Usbeen base "have been more or less separated to limit risk since the Gwan incident", said Captain Frederic, who is only able to tell reporters his first name under French military rules.

A thick, high wall separates the French and Afghan areas, albeit one built before the January killings. Entrance to the French side is guarded by a barrier, barbed wire and armed soldiers. Few Afghans are able to cross.

Afghans also have to check in their weapons once their duties are over and "no longer come to our side, except if they need to see the doctor", said Frederic. In reality, that amounts to a maximum of five visits a day.

French and Afghan soldiers now do sport separately, even at the neighbouring base in Surobi where there are only a handful of local translators.

Joggers have to be accompanied by at least one other soldier and at least one of them has to carry a pistol.

Colonel Gros insisted trust was slowly rebuilding.

"There was always separation, because each side has their habits and way of life. And protection was always there," he told AFP.

"But now the doors are opening. Meetings take place. Contacts take place. I see the (Afghan) battalion head very often, some of my men do as well. So there is no shut-off. Not at all."

Yet only several dozen trainers -- like those killed in January -- come into regular contact with local troops. Three of them told AFP that they were "comfortable" with their mission.

"We can't always look over our shoulders because we mix with the same people. Trust builds over time. We're starting to get to know them. They're really looking to us to protect them," said French Lieutenant Sebastien.

An Afghan military intelligence officer who gave his name only as Rostum said he was "really sorry" about what happened in Kapisa.

"My unit is doing everything possible to make sure it doesn't happen again. I cannot say there are no Taliban infiltrators into our army, but in my unit, I'm convinced of it," he said.

Captain Alexandre said Afghan officers were aware of the dangers and that they were also concerned about their own security within their ranks.

The January 20 shootings did not come as a bolt from the blue. A classified report leaked to The New York Times earlier this year described such "green on blue shootings" as a "systemic" problem.

The report put the killings down to a decade of contempt on each side, and profound ill-will among both civilians and soldiers on both sides. But it downplayed the role of Taliban infiltrators in such incidents.

The distrust between Afghans and foreigners has only deepened since a rogue US soldier was accused of killing 16 villagers, mostly women and children, this month.

For French troops, rebuilding the trust after the January shootings is not easy.

Many were troubled by a rumour that the Afghan identified as carrying out the attack, 21-year-old Abdul Mansour, had time to reload his weapon while his Afghan colleagues stood idly by.

"We're told to say everything's going well," said one young recruit. "But clearly some soldiers don't trust them any more."