A NATO soldier and a civilian contractor have been killed following a suspected insider attack in eastern Afghanistan, bringing to 2,000 the number of US soldiers killed in Afghanistan.
The toll has steadily risen in recent months with a spate of attacks by Afghan army and police against American and NATO troops.
NATO's International Security Assistance Force (ISAF) said in a statement there were also a number of Afghan National Army casualties in Saturday's attack.
It did not specify how many Afghans were killed or wounded.
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Abdul Wali, a local police spokesperson, told the AFP news agency that three Afghan army soldiers had been killed and two others wounded in the incident. He said that three ISAF personnel were also wounded.
The attack took place at an ISAF checkpoint on the Kabul-Kandahar highway in Wardak province, witnesses told Al Jazeera. It occurred at about 5pm local time (12:30 GMT) on Saturday, said General Zahir Azimi, a spokesperson for the Afghan defence ministry.
Witnesses said that gunfire had erupted after a dispute between ISAF soldiers, who were manning the checkpoint, and an Afghan National Army patrol, Al Jazeera's Bernard Smith reported from Kabul.
"[The NATO troops] were searching vehicles [carrying] men, women and children, and an Afghan Army patrol came along the highway [from their own checkpoint]. The Afghan patrol complained that the NATO troops were checking women and children, and it seems as a result of this confrontation a firefight broke out," reported Smith.
The Afghan casualties were "a result of the engagement" on Saturday night, an ISAF spokesperson told the AFP news agency, but could not confirm whether they had been killed by the insider or in return fire by ISAF troops.
A joint Afghan and ISAF investigation into the incident is under way, ISAF said.
The alliance did not specify the nationalities or identities of those killed, saying that such announcements were to be made by the relevant national authorities.
'Mad as hell'
General John Allen, the top US commander in Afghanistan, has said that he is "mad as hell" about attacks by Afghan soldiers on foreign troops, but expected them to continue until a full combat forces pullout is complete in 2014.
Speaking to the US television station CBS in a programme to be aired on Sunday, Allen said: "I'm mad as hell about them, to be honest with you. [...] We're willing to sacrifice a lot for this campaign, but we're not willing to be murdered for it."
Allen added that the "vast majority of Afghans [are] with us in this", according to excerpts of the interview released by the network.
If Saturday's attack was confirmed to have been carried out by a member of the Afghan security services, or by an individual dressed as a member of said force, it would bring the total number of ISAF troops killed in 36 such attacks this year to 52, accounting for about 15 per cent of all coalition casualties in the war.
NATO attributes about 20 per cent of the attacks to infiltration by Taliban fighters into Afghan security forces while the rest are believed to result from cultural differences and personal animosities between the allies.
The so-called green-on-blue attacks pose a serious question to NATO plans , which portrayed the advising and training of Afghan forces as the key to the scheduled pullout of foreign troops.
Earlier this month, ISAF announced a scaling back of joint operations with its Afghan partners following a dramatic rise in such assaults, in which Afghan soldiers turn their weapons on their Western allies.
Allen said that just as homemade bombs had become the signature weapon of the Iraq war, he believed that in Afghanistan, "the signature attack that we're beginning to see is going to be the insider attack".
On Thursday, Leon Panetta, the US defence secretary, announced that ISAF had restarted most joint operations with Afghan forces. It is unclear whether the latest attack will have an impact on those plans, an ISAF spokesperson said.