The commander of an Afghan lieutenant who killed two British soldiers denied Tuesday that the officer was a Taliban infiltrator, saying he had served in the army with credit for years.

Concern is growing about so-called green on blue attacks with 16 NATO troops killed by their Afghan colleagues since January -- more than one in six of the 91 foreign soldiers to have died in Afghanistan so far this year.

The killings threaten to undermine the West's strategy to train Afghans to take over security by the end of 2014, allowing NATO combat troops to withdraw after a long and costly war against the Taliban.

Two British soldiers were shot dead by Lieutenant Gul Nazar in Lashkar Gah, the main town in Helmand province, on Monday. Nazar was also killed. A US soldier was killed on the same day by a local policeman in Paktika province.

The Taliban claimed that the Lashkar Gah attack was in revenge for a recent massacre of civilians, for which a US soldier has been charged with 17 counts of murder, and said one of its fighters had infiltrated the army.

But Nazar's commanding officer, General Sayed Maluk of the Afghan army's 215 corps, denied the claim, saying the lieutenant "was never a suspect, never had any bad records or any links to the Taliban".

He was on the verge of completing his second term of service after about five years in the army, he told AFP.

"During these five years he was with NATO soldiers several times. He was even with them during operations. I strongly reject Taliban claims that he was their man," he said.

Maluk sent the officer to the Lashkar Gah base to collect a commander, he said, "but apparently the NATO soldiers did not open the gate for them, he got angry and shot them".

His account illustrates the cultural differences and resentment driving many of the attacks, which have left seven Americans, four French trainers, two Britons and an Albanian among the dead so far this year.

The US military has also said that most of the attacks are not the result of infiltration by Taliban insurgents.

"From our investigations they have accounted for less than 50 percent," the US commander on the ground, General John Allen, told the Brookings Institution think tank. The assailants were instead "generally self-radicalised", he said.

Allen said the "insider threat" was typical of counter-insurgency warfare, and that US forces faced similar problems in Iraq and Vietnam, and acknowledged the killings had resulted in an "erosion of trust" between the allies.

A recent report commissioned by the US military found deep distrust and suspicion between Afghan and American troops, describing green-on-blue shootings as a "systemic" problem and calling into question NATO's plans.

Such killings "are provoking a crisis of confidence and trust among Westerners" training Afghan national security forces, it said.

The 2011 document said Afghan soldiers described their American comrades as rude, disrespectful and reckless with gunfire when civilians were nearby, while US troops saw Afghan personnel as traitorous, lazy, drug-addled and corrupt.