The Afghan policeman walked into the room without introducing himself, sat down with his rifle in his lap, and stared at the US soldier with a strange look in his eyes.

"It was the most nervous I've been here," said 24-year-old Second Lieutenant Alex Panosian. The surge in insider attacks, in which more than 50 foreigners have been killed by their Afghan colleagues this year, was playing on his mind.

Panosian had been playing cards with Afghan army and police commanders in an attempt to improve sometimes difficult relations at their base in Baraki Barak, Logar province, when the low-ranking officer entered the room.

Panosian told the commanders the situation was making him nervous, at which point they yelled at the Afghan officer to put his weapon away. They then explained that the officer was high on drugs, an all-too-common affliction.

"To them that explained his behaviour, but it made me a little bit more nervous," Panosian said. "It did remind me that even if you're with the chiefs, it only takes one guy."

The unprecedented number of insider killings comes at a critical moment in the 11-year war, as US-led NATO forces try to hand security responsibility to the Afghans ahead of the withdrawal of combat forces by the end of 2014.

Panosian put his nerves down to his "senses being overly alert" because of the attacks. He was at pains to point out the Afghans took the threat seriously and had assured the Americans nothing like that would happen under their watch.

And for the most part in Logar, south of Kabul, US troops say the insider threat is "just another danger of the job", but not one they are particularly concerned about.

Interactions take place mostly at the officer level and joint patrols, scaled back in the wake of the insider attacks, have not restarted in Baraki Barak, a Taliban stronghold.

US soldiers do take precautions, however, with so-called guardian angels keeping a close eye on their local partners when visiting the Afghan side of the base -- trying to be as inconspicuous as they can while carrying a loaded automatic rifle.

On September 30, a firefight in neighbouring Wardak province between NATO troops and their Afghan allies killed five people, including one US soldier and a civilian contractor, in a suspected insider incident which is still under investigation.

The psychologist for the 173rd Airborne Brigade Combat Team, Captain Julie Welch, who travelled to the base in Sayedabad the next day to provide counselling, said such attacks made the soldiers question the Afghan-US partnership.

"It affects them a little differently from an IED (improvised explosive device) blast, gunshot wound or suicide blast, because it comes along with trust issues, questioning partnership and questioning trust in other people.

"Trust issues lead more to them relying on each other. It brings them closer together as a unit and it leads them into a very close-knit group. They turn more to each other."

She insisted the soldiers were professional, able to grieve and then re-focus on the mission, but said: "When you're constantly alert, constantly on guard, that can lead to more anxiety.

"Out here it leads to more hyper-vigilance, alertness, which can sometimes manifest in sleep issues, because when you're alert all the time it's hard to calm yourself down before bedtime."

One civilian police trainer, who did not want to be named, said despite having a good relationship with the Afghans, he always kept a hand on his gun and practised drawing his pistol in his bedroom.

"I respect them but I'm ready. When I go into a room I'm always going through it in my mind what to do, use this one as a shield, shoot that one," he said.

"I had a good relationship with them but I didn't ever know what damage was done elsewhere. It's unpredictable, so I always kept my gun ready with one (bullet) in the pipe."

The NATO force has suggested 25 percent of the attacks stem from the Taliban insurgency and the rest from arguments and cultural misunderstandings.

"If you're the enemy it's a good tactic," said Colonel Andrew Rohling, the commander of the 173rd Airborne Brigade Combat Team. "From the enemy standpoint it can drive a wedge between us and the Afghan army which is significant."

The Afghans are trying to address the threat themselves, with intelligence agents in the ranks and a brochure telling soldiers not to be offended if, for example, a US counterpart blows his nose in front of them.

"It's hard to completely not be American," Rohling said.

"Guys aren't doing it to make the Afghans mad. But the difference sometimes here in Afghanistan is you resolve issues with a gun instead of resolving them by talking."