It's called the "friendship wall", but when US soldiers pass through the gate from their base to the Afghan side they "go red", loading their weapons with a round in the chamber in case of attack.

Commanders say relations between American and Afghan troops are good at Combat Outpost McClain in Logar province, just south of Kabul and a key battleground for preventing insurgent attacks on the capital.

But they are working to mitigate one of the worst scourges of the 11-year war -- so-called insider attacks that have seen more than 50 Western soldiers shot dead by their Afghan allies so far this year.

NATO attributes about 25 percent of the attacks to infiltration by Taliban insurgents into Afghan security forces, while the rest are believed to result from cultural differences and personal animosities.

The unprecedented scale of killings threatens to derail NATO's carefully laid plans to withdraw almost all combat troops by the end of 2014, leaving Afghan troops to fight the Taliban.

Despite upbeat forecasts from NATO and Western officials, the International Crisis Group warns that the Afghan government could fall apart after 2014 and that Afghan forces were "overwhelmed and underprepared" for the transition.

COP McClain commanders deny the wall indicates division and mistrust.

They say the Afghan army is increasingly independent and that thanks to close relations, the Afghans understand why the Americans take the precaution of carrying a loaded weapon.

"They don't see it as anything disrespectful," said Major Matthew Albertus, the US officer in charge of COP McClain and northern Logar, 173rd Airborne Brigade Combat Team.

"Lieutenant Colonel Rafiullah takes it as a personal matter of pride to make sure his advisors are taken care of. He feels responsible for our safety."

Rafiullah's was one of 74 battalions out of 156 rated in the second-best category of "effective with advisors" in an April report by the US Department of Defense.

Just 13 Kandaks, or battalions, were in the top category, "independent with advisors", fuelling widespread concerns that the 350,000-strong Afghan force will not be able to withstand the Taliban post-2014.

Lieutenant Colonel James Wright, commander of 1st Squadron (Airborne) 91st Cavalry Regiment, 173rd, said the 7th Kandak was "probably the best we've got in the province", while other units were struggling in areas such as leadership.

"We're obviously trying to build an army that's going to be victorious in battle, but the reality is these guys have got to be just that much better than the insurgents," he said.

"We have that with 7th Kandak. We're not quite there in areas of the south, where there have been dramatic increases in the number of foreign fighters for instance."

The decision to build the wall was made in mid-August, the month that saw the highest number of insider attacks in Afghanistan.

Rafiullah, who goes by one name, felt it would give the Afghans their own camp with its own entrance, making them feel more independent.

He requested that 25 of his officers be issued with ID cards to travel freely back and forth -- unarmed -- to the US side.

"We're able to talk about this stuff, it's not the elephant in the room," said Albertus.

Rafiullah's spokesman, Captain Hayauddin Hekmat, said Afghan forces are tackling the potential threat in other ways too, with intelligence agents in Afghan army uniform working among the ordinary soldiers.

"For four months we've been doing combined missions without incident," he said.

Out on patrol among fields, streams and orchards, Platoon Sergeant Jason Patrick, of 1-91 CAV, a straight-talking 35-year-old on his third tour of Afghanistan, summed up his own viewpoint.

"You get good ANA (Afghan National Army) and bad ANA. Sometimes they suck, sometimes they want to make chai (tea) in the middle of a patrol," he said. "But our ANA here, they are stellar to work with. They are ready."

Last month, NATO scaled back joint operations with Afghans below battalion level, although US Defense Secretary Leon Panetta has since said that most operations have resumed normally.

After discovering a 107mm rocket buried in tall grass, the 1-91 CAV platoon called in the ANA, fearing the device might be booby trapped.

When the Afghans arrived, the US platoon had its "Guardian Angel" system in place, with soldiers keeping watch unobtrusively, weapons ready. But the platoon leaders from both sides greeted each other warmly with handshakes and smiles.

US commanders say they are teaching the Afghans what they can, but then it will be up to them. Ultimately they will take on the challenge in an Afghan way.

As if to emphasise the point, Afghan platoon leader Ahmad Jabryal, a 29-year-old ethnic-Tajik, took a quick look at the rocket and picked it up with his bare hands with a half-smile.

"It's empty except for dirt inside -- and some mushrooms," he said. "I've seen this type of rocket many times before."