General Martin Dempsey, the chairman of the US joint chiefs of staff committee, has termed the increasing occurrence of "insider attacks" by Afghan security forces on foreign troops "a very serious threat" to the war effort in that country.
Speaking to the Pentagon's press service after the latest such attacks on Sunday, which killed four US soldiers and wounded two others, he said: "You can't whitewash it. We can't convince ourselves that we just have to work harder to get through it. Something has to change."
The top US military officer was in Romania for talks with NATO officials about the so-called green-on-blue attacks, which have claimed 51 lives this year - up from 35 in 2011.
Dempsey said that the Afghan government needed to take the problem as seriously as US commanders and officials.
Speaking on Monday, Leon Panetta, US defence secretary, termed such attacks the "last gasp" of a Taliban campaign to regain lost ground.
The four US soldiers and an Afghan police officer were killed in southern Afghanistan on Sunday following an attack suspected to involve members of the Afghan police, NATO's military mission in that country said.
The attack took place at a checkpoint in the Mizan district of Zabul province, and was carried out by several Afghan men dressed in police uniforms, the deputy governor's office told Al Jazeera.
The four soldiers were found dead and two wounded when a response team arrived at the scene from a nearby checkpoint, a spokesman for the coalition said.
One of the six members of the Afghan National Police (ANP) operating the observation post with six coalition troops was also found dead, while the other five had disappeared. "The fighting had stopped by the time the responders arrived," Major Adam Wojack, a spokesman for the NATO-led coalition, said.
'Insider attacks' rising
It was unclear if the police officer who was killed was one of the attackers, Al Jazeera's Bernard Smith in Kabul reported.
ISAF said that the attack was "under investigation".
All six of the dead and wounded foreign troops were understood to be members of the US special forces, Smith reported.
Afghanistan's defence ministry said earlier this month that it had arrested or sacked hundreds of Afghan soldiers for suspected links to the Taliban or other anti-state fighters.
Afghan and NATO officials say, however, that about 75 per cent of the attacks are not connected to the Taliban and are mostly triggered by misunderstandings and cultural differences among the Afghans and their Western allies.
Kate Clark, a Kabul-based analyst with the Afghanistan Analysts Network (AAN), told Al Jazeera that insider attacks "strike right at the heart of what ISAF and NATO are trying to do [in Afghanistan]".
She said that the scale of the coalition's training effort for the Afghan security forces creates definite vulnerabilities.
"The very speed of that training programme, the vastness of it [with] tens of thousands of men being trained, means that I think it's been difficult to keep a handle on it. It's a vulnerable place where people with malintent can get close to the foreign forces," she said.
Shukria Barakzai, an Afghan minister of parliament and the head of the Parliamentary Defence Committee, told Al Jazeera that the attack would reduce confidence among ISAF and NATO troops.
"In the meantime, the [NATO] troops that have to train the Afghan troops face huge difficulties ... How can they
continue the training process when they don't have enough confidence from those Afghan soldiers," she said.
Barakzai said that Afghanistan faced many challenges, that would go beyond NATO's 2014 withdrawal deadline.
"It's not only about the Afghan soldiers' appropriation ... They don't have enough equipment, they don't have enough the proper agenda ... It means we [cannot] control the challenges and problems the Afghan national security force are facing, and the regional countries are facing."
In a separate development, at least eight women and children were killed in a NATO air attack in the Alingar district of Laghman province, local officials said on Sunday.
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Sarhadi Zewak, Laghman provincial government spokesman, said villagers from the district brought the bodies to the governor's office on Sunday.
"They were shouting 'Death to America!' They were condemning the attack," he said.
The women were out picking nuts at the time of the attack, late on Saturday night, a provincial council member's office told Al Jazeera.
Latif Qayumi, the provincial health director, said that seven injured women were brought to the hospitals for treatment, some of them as young as 10 years old.
NATO confirmed that a strike had taken place in the area and acknowledged that civilian deaths had occurred.
"ISAF takes full responsibility for this tragedy," a statement said. The military alliance insisted that fighters had been the target of the strike.