Four NATO soldiers were killed Sunday in the second suspected attack by Afghan police in two days, as officials detailed unprecedented damage from a Taliban assault on the base where Britain's Prince Harry is deployed.
Afghan authorities said the shooting took place in Zabul province, part of the restive south where the more than 10-year Taliban insurgency is traditionally strongest, but the Islamist militia denied responsibility.
The US-led International Security Assistance Force (ISAF) released few details other than that it was "suspected to involve members of the Afghan police".
Ghulam Jilani, the deputy police chief in Zabul, said a police post in Mizana district came under attack, but that after NATO troops arrived to help, a policeman opened fire on the Westerners.
A senior provincial official confirmed that NATO forces came under fire, and that the Westerners returned fire, killing one policeman.
"Three to four other policemen have disappeared. At the moment, we don't know where they have gone. We don't know if they fled fearing arrest or if they are linked to the Taliban," he told AFP on condition of anonymity.
Taliban spokesman Yousuf Ahmadi said those responsible were not infiltrators. "He was just an Afghan who did this out goodwill," he said.
Sunday's deaths took to 51 the number of Western soldiers killed by their Afghan colleagues since the start of the year, in a growing trend that jeopardises NATO plans to train local forces to take over when they leave.
Two British soldiers were killed in a similar attack on Saturday in the southern province of Helmand. The British defence ministry said they were shot by a man wearing the uniform of the Afghan Local Police (ALP) at a checkpoint.
This month, US special forces suspended training for about 1,000 recruits to the controversial unit, which fights in remote areas of the countryside. It has also been accused of corruption and violence towards civilians.
Afghanistan says it has arrested or sacked hundreds of Afghan soldiers for suspected insurgency links in a bid to stem the so-called insider attacks.
NATO attributes around 75 percent of the attacks to grudges, misunderstandings and cultural differences. The Afghan defence ministry this month published a brochure for the Afghan army with advice on how not to misunderstand Westerners.
The military on Sunday detailed unprecedented damage from a sophisticated, well-coordinated attack on one of the largest NATO bases in the country, Camp Bastion where Britain's third in line to the throne is deployed.
Two US Marines were killed and several others wounded in the assault late Friday, which was carried out by at least 15 attackers dressed in US Army uniforms and armed with guns, rockets and suicide vests.
NATO on Sunday confirmed the material losses as six US AV-8B Harrier fighter jets destroyed and two significantly damaged, three coalition refueling stations destroyed and six aircraft hangars damaged.
"The insurgents appeared to be well equipped, trained and rehearsed," said ISAF in the statement.
The militia claimed the assault was to avenge a US-made film deemed insulting to Islam that has sparked deadly riots across the Middle East and North Africa.
Prince Harry was never in danger, officials confirmed. Although the Taliban have vowed to kill the third in line to the British throne, one of its spokesmen told AFP that the assault "had nothing to do with the prince".
The attack raises serious questions about how insurgents managed to penetrate such a massive logistics hub in the desert, home to 28,000 soldiers.
On Sunday, the military also announced the arrest of a purported Taliban leader, who it said was responsible for bringing down a helicopter that killed two American troops on September 5.
The incident happened in Logar province, south of Kabul, in an area largely under Taliban control and the militia claimed responsibility. At the time, NATO said the cause of the crash was under investigation.
At least 332 Western troops have been killed in Afghanistan so far this year, according to the iCasualties website, 253 of them American.
NATO has 112,579 troops currently in Afghanistan. The Pentagon said last week that there are 77,000 US troops in the country.