Three foreign soldiers, two of them British, have been killed by Afghan security personnel in two separate shootings, bringing the number of such deaths to 16 this year.
An Afghan soldier said to be an officer shot dead the Britons in the southern province of Helmand, while the NATO-led International Security Assistance Force said an "alleged member" of the police killed a trooper in eastern Afghanistan.
More than one in six of the 91 foreign soldiers to have died in the country in 2012 has been killed in so-called "green-on-blue" attacks by Afghan security personnel, significantly raising tensions between NATO forces and their local colleagues.
"It appears that a member of the Afghan national army opened fire at the entrance gate to the British headquarters in Lashkar Gah city, killing the two British service personnel," said Britain's Defence Secretary, Philip Hammond.
The attacker was shot dead by coalition forces and officials said another British soldier was severely wounded.
Lashkar Gah is the main town in Helmand, a Taliban stronghold, but was among the first places where security responsibility was handed from ISAF to Afghan forces as part of a gradual transition process.
Provincial police chief Abdul Nabi Elham said the gunman was a lieutenant named Gul Nazar from Jalalabad.
"As soon as the NATO soldiers opened the game for him and his team in their centre, this soldier opened fire at them and killed them," he said.
"We don't know the motive behind this attack and have not found a link to the Taliban. We are still investigating."
A spokesman for the militants contacted by AFP described the gunman as being "in contact" with them, although he did not claim the attack as being mounted by the Taliban.
"Today an army officer who was in contact with us killed three NATO soldiers in Helmand," said Qari Yousif Ahmadi by telephone from an undisclosed location.
The Taliban regularly exaggerate their claims.
Exact details of the incident remained unclear, with accounts differing on whether an argument took place before the shooting started, and whether another Afghan soldier was killed.
The shooting comes less than three weeks after six British soldiers were killed when a huge explosion ripped through their armoured vehicle near Lashkar Gah.
Taliban insurgents claimed responsibility for that attack, which pushed the number of British dead in Afghanistan over the 400 mark.
ISAF said in a statement that in the second incident its service member was "shot by an alleged member of the Afghan local police as the security force approached an ALP checkpoint".
It did not state the victim's nationality, in line with policy.
A spokesman said the incident happened in Paktika province and the fate of the gunman was unclear. Initial reports indicated two "alleged shooters" were involved, he said.
Monday's deaths brought the number of ISAF service members killed by Afghan security personnel this year to 16.
The US-led NATO force is training Afghanistan's own units to take over national security by the end of 2014, allowing foreign combat troops to withdraw after a costly and lengthy war against the Taliban insurgency.
But a classified coalition report leaked to The New York Times earlier this year described green-on-blue shootings as a "systemic" problem.
The report put the killings down to a decade of deep-seated animosity on each side, and profound ill-will among both civilians and soldiers on both sides, downplaying the role of possible Taliban infiltrators in such incidents.