An embarrassing statistical error has raised serious questions about the progress Western forces are making towards improving security in Afghanistan.

NATO's International Security Assistance Force (ISAF) have admitted to incorrectly reporting a 7 per cent drop in the number of attacks by Taliban insurgents last year.

ISAF says it was a record-keeping error that failed to take into account numbers from Afghan forces that are increasingly taking a bigger role in the conflict as Western forces pull back.

Bill Roggio, a commentator on military affairs in Afghanistan, is confident the miscalculated statistic was not an intentional attempt to "fudge numbers".

"I think this is a result of is trying to make these numbers look good but when you go back and look at the numbers, even the numbers that they provided don't," he said.

"What you see is a spike in attacks pre-surge and then they level off but ISAF has wanted to show a couple of per cent drop here and there as a major success.

"I'm not going to accuse them of fudging the numbers intentionally."

In December 2009, US president Barack Obama sent an additional 30,000 troops to Afghanistan hoping to stabilise the nation and quell what was then widely viewed as a Taliban resurgence.

Critics of US policy in Afghanistan say the botched statistics prove the surge was ineffective.

But with the bulk of Western troops being withdrawn by the end of next year, many analysts, including Mr Roggio, are questioning the ability of local security forces to hold back the Taliban.

"You know, I don't see any chance that the Taliban are just going to go away now that Afghan security force is in," he said.

"In fact, I think the Taliban are going to redouble efforts.

"The Afghan security forces can't sustain themselves in the field and they don't have the fighting capability of Western forces."

The most recent insider attack in the war-torn country happened as on Wednesday when an Afghan police officer drugged and then shot dead 17 of his colleagues.

On the same day, a suicide bomber injured six Afghan soldiers and a civilian in an attack on a bus in Kabul.

While there are questions about the progress being made, the Pentagon maintains the broad trends in Afghanistan are heading in the right direction

It says 80 per cent of the violence occurs in areas where less than 20 per cent of the population lives.