Afghanistan said Wednesday it had arrested or sacked hundreds of Afghan soldiers in a bid to stem a rise in insider attacks on NATO forces, a trend that threatens to undermine Western plans for a troop withdrawal.

The surge of assaults, unprecedented in modern warfare, have seen Afghan troops opening fire on their NATO colleagues more than 30 times this year, killing at least 45 foreign troops -- most of them Americans.

The shootings have raised questions about the rapid growth of the Afghan army and police and alarmed alliance commanders, who have been under pressure to expand the Afghan forces to pave the way for an eventual exit of Western troops.

Defence ministry spokesman General Zahir Azimi said Wednesday: "So far, hundreds of people have either been arrested or expelled from the army. We have found evidence against some people and some suspicious people have been arrested."

When asked for further details, Azimi gave no breakdown on precise numbers. Nor was it clear when the action was taken against the soldiers.

US Lieutenant General James Terry, head of the NATO-led coalition's joint command in Afghanistan, said he heard 200-300 Afghan troops were removed from the force but was waiting to learn precise numbers and details from Afghan defence officials.

The Afghan government was expected to launch a "counterintelligence initiative" soon to pre-empt attacks and identify potential turncoat threats within army and police units, Terry told reporters in Washington via video link from Afghanistan.

He suggested the Afghans would review screening for the whole security force, which is due to reach 352,000 next month, and acknowledged the effort would take time.

"And in relationship to vetting the entire force, I'm sure it will take considerable time, but I think what the Afghans want to do is be very sure of their process and then go back and re-check," he said.

But he predicted the security screening would not "slow down the pace of operations."

The attacks pose a threat to the linchpin of NATO's strategy of training Afghan forces to take over when the bulk of the 130,000 US-led foreign troops leave the country at the end of 2014.

On Sunday, the US military announced that its special forces have suspended training for about 1,000 Afghan police recruits to vet existing members.

Karzai's spokesman told AFP on the same day that the attacks were the "mutual responsibility" of both NATO and Afghan forces, and the president had ordered all Afghan forces to be re-vetted.

Azimi on Wednesday denied that NATO training for Afghan soldiers had been affected, with the spokesman insisting it was still "going well".

He also stood behind the processes of recruitment and vetting.

"Good attention was paid during the recruitment process, but when some soldiers went on vacation and came back they became somehow problematic," he said.

NATO has tried to play down the attacks, saying that they are carried out by a tiny proportion of the Afghan forces over cultural differences or personal disputes.

The alliance's Secretary General Anders Fogh Rasmussen on Wednesday spoke about the attacks by telephone with Karzai, voicing his concern and calling on the president "to join in" with preventive measures being taken by NATO.

Taliban insurgents claim responsibility for many of the attacks, saying their fighters have infiltrated the army and police.

Their reclusive supreme leader, Mullah Omar, has boasted that the attacks are the result of a deliberate plan to sow distrust between foreign and Afghan troops.

An Afghan army officer in the restive eastern province of Paktika blamed Taliban infiltrators and poor treatment from US mentors.

"Most of our soldiers come from different provinces, they are illiterate, and US mentors sometimes behave badly with them. That is why they turn their weapons at them," he told AFP on condition of anonymity.

August was the worst month for so-called green-on-blue attacks in Afghanistan, with nearly one in three international coalition deaths caused by Afghan allies.