WASHINGTON: The US general leading Nato forces in Afghanistan acknowledged on Thursday that the Taliban could be traced to more “insider attacks” against Western troops than previously acknowledged, accounting for about a quarter of the cases.

The increasing number of killings of Nato soldiers by Afghan security forces, or those impersonating them, have eroded trust between Western forces and their Afghan allies and threaten to complicate plans for a transition to Afghan security within two years.

Last week, the Pentagon, citing Nato data, said only about 11 per cent of so-called “insider attacks” by Afghans against Nato troops are due to Taliban infiltration, with the rest caused by other motives, such as personal grudges.

But Marine General John Allen said the figure was actually closer to 25 per cent.

“Our view is it's about 25 per cent,” Allen said. “If it's just pure Taliban infiltration, that is one number. If you add to that impersonation, the potential that someone is pulling the trigger because the Taliban have coerced the family members, that's a different number.”

Allen's Nato-led force later issued a clarification, suggesting that Allen's data and the 11 per cent figure did not contradict each other, however it did not provide a year-by-year breakdown.

It said Allen was referring to data going back to 2007. The lower figure released last week looked only at insider attacks that occurred so far in 2012, Nato said in a statement distributed by the Pentagon.

There have been 32 insider attacks so far this year involving 36 shooters that have led to 40 coalition deaths, with 25 of them Americans. Some 69 coalition troops have been wounded. That is a sharp increase from 2011, when, during the whole year, 35 coalition troops were killed in such attacks, 24 of whom were US troops.

Many of the attacks have been claimed by the Taliban as evidence of insurgent reach and infiltration.

Allen declined comment on Afghan claims that foreign spies were the biggest culprits.

In a video briefing from Kabul, he said he wanted to see the intelligence supporting remarks by Afghan President Hamid Karzai's office that foreign spies were behind most of the attacks, including those from “neighboring countries” - an allusion to Pakistan and Iran.

“I'm looking forward to Afghanistan providing us with the intelligence that permits them to come to that conclusion,” Allen said, adding he “could add that into our analysis.”

Allen said he could not provide his opinion about the claim until he saw that intelligence. But his comments did not appear to endorse the Afghan government's view. Allen insisted the attacks defied simple explanation, saying “the reasons for these attacks are very complex.”

Allen said he did not believe the spate of insider attacks should prompt American forces to pull back in their contact with Afghans “at this juncture.”

Nato commanders have largely played down the threat of infiltration, blaming most of the shootings on stress or personal differences between Afghans and their Western advisers that ended at the point of a gun, a frequent occurrence in Afghanistan.

The Pentagon announced last week it was expanding counterintelligence staff in Afghanistan and the Afghan government said on Wednesday it would re-examine the files of 350,000 of its soldiers and police to help curb rogue shootings of Nato personnel.