KABUL  - Afghanistan said Wednesday it had arrested or sacked hundreds of Afghan soldiers over attacks on Nato troops, trying to stem an increase in shootings that threatens to undermine Western withdrawal plans.

The scale of attacks is unprecedented in modern warfare. Afghan personnel have opened fire on their Nato colleagues more than 30 times so far this year, killing at least 45 foreign troops - the majority of them American.

The shootings threaten to derail Nato’s flagship strategy of training Afghan security forces to take over when the bulk of the 130,000 US-led foreign troops leave the country at the end of 2014. US officials have expressed increasing concern over the attacks and Defence Secretary Leon Panetta last month called on Afghan President Hamid Karzai to crack down on them.

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.

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.

The top US military officer visited Afghanistan last month and said Afghan leaders appeared ready to take decisive action to curb the attacks.

General Martin Dempsey, chairman of the US Joint Chiefs of Staff, said it was the first time that Afghans were as concerned about the attacks as the Americans. 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. 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.

Meanwhile, a Nato helicopter crashed in Afghanistan on Wednesday, killing two US troops in an area south of Kabul that is largely under Taliban control, officials said.

The militia leading a decade-long insurgency against foreign troops claimed to have shot down two helicopters in Logar province, but Nato said the cause of the crash was under investigation.

The militants are known to make exaggerated claims.

“An ISAF helicopter has crashed in eastern Afghanistan, resulting in two ISAF fatalities. We are investigating,” a spokesman for Nato’s US-led International Security Assistance Force told AFP.

In Washington, a US defence official, who spoke on condition of anonymity, later said the two troops killed were Americans and that the aircraft that went down was a small, single-engine OH-58 helicopter.

The administration in Logar said the area, 13 kilometres north of the provincial capital Puli Alam, is mostly controlled by the Taliban.

“There was a helicopter incident in Baboos area, which is an area largely under Taliban control,” said provincial spokesman Din Mohammad Darwish. The area was “inaccessible” to Afghan forces, he added.

In a statement on their website, the Taliban claimed to have used rockets to shoot down two American helicopters, killing “tens of invaders”.

Logar has become an increasing flashpoint for violence.

On Wednesday, defence ministry spokesman, Zahir Azimi, admitted that security challenges have increased in Logar and neighbouring province Wardak, but said there was a plan to deal with it.

ISAF had earlier on Wednesday announced that a Taliban leader wanted for an August 7 truck bombing of a Nato base in Logar had been killed in an air strike in Puli Alam on Tuesday.

The military said it believed the Taliban operative had been planning another large-scale attack.

Helicopter crashes are fairly frequent in Afghanistan, where the 130,000-strong Nato mission relies heavily on air transport.