A US soldier and a civilian contractor have been killed in Afghanistan by an Afghan soldier, bringing the total number of US soldier deaths in the 11-year-old conflict to 2,000.
A US official, who asked not to be identified, told Reuters news agency on Sunday that an American soldier and a civilian contractor had been killed in the incident in eastern Afghanistan.
The attack happened at about 12:30 GMT on Saturday in the Sayd Abad district just outside a joint US-Afghan base in Wardak province, an Afghan defence ministry spokesman said on Sunday.
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NATO forces would only say that it was a "suspected insider attack" and that a NATO service member and civilian contractor were killed.
Saturday's attack brings 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.
Al Jazeera's Bernard Smith, reporting from Kabul, quoted witnesses as saying that gunfire erupted after a dispute between International Security Assistance Force (ISAF) soldiers, who were manning the checkpoint, and an Afghan National Army patrol.
"[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," he said.
"It's a particularly grim day for the American forces because that death brings to two thousand the number of troops killed here in Afghanistan in more than ten years."
So-called insider attacks are now among the biggest threats to coalition troops.
Josh Earnest, deputy spokesman to President Barack Obama, said that top US officials were aware of the details of the latest attack.
"Make no mistake ... these attacks do not diminish in any way the commitment of the president, the commitment of our men and women in uniform or the commitment of our allies to follow through and complete successfully the mission to end the war in Afghanistan in 2014," Earnest said.
He said the US and its allies had taken a number of steps to mitigate the risk of insider attacks, including greater vetting of Afghan forces.
The toll has steadily risen in recent months with a spate of attacks by Afghan army and police - supposed allies - against US and NATO troops.
The attacks raise questions about whether countries in the US-led coalition in Afghanistan will achieve their aim of helping the government in Kabul and its forces stand on their own after most foreign troops pull out in little more than two years.
According to the Afghanistan index kept by the Washington-based Brookings Institution, about 40 per cent of the American deaths were caused by improvised explosive devices.
The majority of those were after 2009, when Obama ordered a surge that sent in 33,000 additional troops to combat heightened Taliban activity.
The surge brought the total number of US troops to 101,000, the peak for the entire war.
"There are concerns that despite all NATO's efforts to reduce insider attacks, what it really needs is for the Afghan security forces, the Afghan military to unite in strong condemnation of these attacks," our correspondent said.
He said though that in some parts of the Afghan military there is no desire to see NATO forces still in Afghanistan.
"That sort of attitude is permeating to troops lower down the ranks," he said.
Mark Kimmitt, a former US assistant secretary of state, told Al Jazeera that he expected to see more incidents of insider attacks in Afghanistan.
"It is clear that the insurgents see this as a valuable way, not only to drive a wedge between the Afghans and the coalition forces, but more importantly to drive a wedge between the coalition forces and the nations from where they come," he said.
"The milestone of 2,000 is bad enough, but the fact that this was done by one of our own allies is not only galling, but it is a sense of betrayal and I think that the [US] public will see it that way."
'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."
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Allen said the "vast majority of Afghans [are] with us in this", according to excerpts of the interview released by the network.
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".
Leon Panetta, the US defence secretary, announced on Thursday 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.