Four American soldiers and an Afghan police officer have been killed in southern Afghanistan following an attack suspected to involve members of the Afghan police, NATO's military mission in that country says.
The latest "insider" attack took place early on Sunday morning, the International Security Assistance Force (ISAF) said in a statement.
General Martin Dempsey, the chairman of the US Joint Chiefs of Staff Committee, termed the threat of such attacks "a very serious threat" to the war effort.
Dempsey said the Afghan government needs to take the problem as seriously as do US commanders and officials.
"We're all seized with [the] problem,'' said Dempsey, after discussing the issue at a meeting in Romania with NATO officials. "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.''
Sunday's 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," said Major Adam Wojack, a spokesman for the NATO-led coalition.
"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.
At least 51 foreign military personnel have been killed in insider attacks, where men dressed as members of the Afghan security forces have attacked foreign forces, in Afghanistan this year.
Two British soldiers died in a similar attack on Saturday in the southern province of Helmand.
There have been more than 30 such attacks so far this year. Most of the casualties have been US soldiers.
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]".
"I think as such the Taliban are one part of the problem. Mullah Omar, in a recent message marking the occasion of the end of Ramadan, actually called on his fighters to specifically target the foreign military in this way. But there's also the attacks arising from the friction between people from different nations fighting or training together," she said.
She said that the scale of the coalition's training effort for the Afghan security forces creates definite vulnerabilities.
"And 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."
Eight civilians killed
Meanwhile, at least eight women and children were killed in a NATO airstrike in the Alingar district of Laghman province, local officials said on Sunday.
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Laghman provincial government spokesman Sarhadi Zewak 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 strike, 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.
"Protecting Afghan lives is the cornerstone of our mission and it saddens us when we learn that our action might have unintentionally harmed civilians," said Jamie Graybeal, an ISAF spokesperson.
Helmand base attack
On Saturday, the Taliban claimed responsibility for a separate attack in southern Afghanistan that left two US soldiers dead and some others wounded on Friday.
Six US fighter jets were destroyed and two significantly damaged during the attack on Camp Bastion, also in Helmand province, NATO spokesperson Lieutenant Colonel Hagen Messer said.
Three coalition refuelling stations were also destroyed and six aircraft hangars damaged in the assault, the US-led NATO force said.
In a statement, it said the attack was "well-coordinated" and carried out by around 15 fighters, who were organised into three teams and who penetrated the perimeter fence.
"The insurgents appeared to be well equipped, trained and rehearsed," targeting fighter jets and helicopters parked next to the runway, ISAF said in a statement released nearly 36 hours after the assault began.
The Taliban said the attack on Camp Bastion, a British airbase, was in response to the anti-Islamic video that has sparked a wave of ongoing protests, Al Jazeera's Afghanistan correspondent Bernard Smith reported from Kabul.
Camp Bastion is adjacent to Camp Leatherneck, the main base for the US Marine Corps in Helmand.
A defence official in Washington, speaking on condition of anonymity, said the two people killed on Friday were US marines, while another US official described the attack as "complex", meaning it was a coordinated strike using several types of weapons.
The attack, involving small arms and mortar or rocket fire, started around midnight local time, Master Sergeant Bob Barko of ISAF told AFP news agency.
Clark, the Kabul-based analyst, termed the attack "a huge blow for NATO".
"This war is not so much about fighting a lot of the time - strange as that may sound - it's a lot about being seen to be winning. A lot of it is about the war of narratives, and this is a blow for NATO and absolutely a victory for the Taliban to get this amount of weaponry destroyed or damaged, and to actually penetrate one of the most highly defended places in the country," she told Al Jazeera.