Four special forces soldiers with the Nato-led force in Afghanistan have been killed in an attack by suspected Afghan police, senior Afghan officials say.
Five police were missing after the checkpoint attack in southern Zabul province, they said. One was killed.
The incident brought to 51 the number of Nato troops killed in "insider attacks" this year.
The latest incident comes a day after two UK soldiers were killed at a checkpoint by a man in police uniform.
On Friday, two US marines were killed in a Taliban attack on Nato's Camp Bastion base in southern Afghanistan.
Militants breached the perimeter of the sprawling Camp Bastion base in Helmand province, destroying six US Harrier aircraft.
The special forces members were killed as they tried to stop an insurgent attack on a police checkpoint in Mezan, a remote area of Zabul near the border with Pakistan, said Zabul deputy police chief Ghulam Jelani Farahiof.
They were killed by a man in an Afghan national police uniform, he said.
Their nationality has not been released but they were not from the UK contingent.
Later, senior Afghan officials told the BBC that the four dead were special forces members.
They said one suspected policeman had died in the attack and five had gone missing since. But it was not clear whether the five were involved in the attack against Special Forces or had fled fearing they would be held responsible.
But a Nato official told Associated Press that an assailant was at large and it was unclear whether there had been multiple attackers.
A number of international troops were wounded in the attack, reports said. An investigation has been launched.
If confirmed, this attack takes the number of Nato soldiers killed in insider - or so-called "green-on-blue" - attacks to 47 for this year alone.
They risk undermining the programme to train Afghan forces, correspondents say.
The rash of attacks prompted the US to suspend training for new recruits to the Afghan local police (ALP) earlier this month.
On Saturday, two British soldiers were shot dead in Helmand province by a man wearing a local police uniform, the UK defence ministry said.
Some 700 members of the Afghan security forces have been ejected in the past month as part of increasingly thorough check being put into place by authorities, according to Afghan officials.
But ultimately, it is very hard to guarantee against this type of killing, says the BBC's Jonathan Beale in Kabul.
More details have emerged about the attack on Camp Bastion, which the Taliban said was in retaliation for a film mocking Islam made in the US.
In a statement, Nato said the attack had been carried out by 15 insurgents dressed in US Army uniforms who "appeared to be well-equipped, trained and rehearsed".
"The insurgents, organised into three teams, penetrated at one point of the perimeter fence," it said.
As well as the six AV-8B Harrier jets destroyed, two were significantly damaged. Three refuelling stations were also destroyed, and six aircraft hangars were damaged.
Fourteen of the insurgents were killed, Nato said, and one was taken into custody. Nine coalition personnel were wounded.