Two separate gun attacks on Nato-led troops in south Afghanistan have left a total of six soldiers dead, coalition officials say.
An Afghan civilian employee shot dead three soldiers, whose nationalities were not given, at a base in Helmand province on Friday.
On the same day, also in Helmand, an Afghan police officer shot three US marines after inviting them to dinner.
The killing of two British soldiers in Helmand was also announced on Friday.
There is mounting concern over attacks on Nato troops by their Afghan allies.
Members of the Afghan security forces have killed 34 international coalition soldiers this year, in 26 incidents, US military spokeswoman Maj Lori Hodge told the BBC.
Last year, there were 35 deaths, in 21 incidents, she said.
In spite of the gains made by coalition forces, and their attempts to wipe out militant leaders, almost every day brings proof that Taliban militants retain the capability to kill, the BBC's Aleem Maqbool reports from Islamabad, in neighbouring Pakistan.
'No reflection'
The Afghan civilian employee was detained and is currently in the custody of the International Security Assistance Force (Isaf), Maj Hodge said.She said the attack had happened at a base shared by Afghan soldiers and Isaf troops, and described the attacker as an "Afghan working on an installation".
However, an Afghan official told BBC News in Kabul that the gunman was a rogue Afghan policeman who had opened fire on Friday evening during a row with foreign soldiers over access to a gym at the base.
In Friday's other shooting, the police commander killed the three marines as they ate before dawn, in observance of the Muslim fasting season, then fled into the darkness.
A US defence department official confirmed that the dead Americans were Marine Special Operations Forces, the Associated Press news agency reports.
Taliban spokesman Qari Yousef Ahmadi told AP by telephone that the attacker had joined the insurgency after the shooting.
"Now, he is with us," the spokesman said.
Writing earlier on Twitter, an Isaf spokesperson said most attacks by members of the Afghan security forces on its personnel were caused by "personal grievances or stress".
"Those incidents do not reflect the overall situation in [Afghanistan], where hundreds of thousands of soldiers, Isaf and [Afghan], work together.
A British soldier from 3rd Battalion The Rifles was killed by "enemy action" while on patrol in the Nad-e-Ali district of Helmand on Thursday.
A soldier from 30 Armoured Engineer Squadron, 26 Engineer Regiment, died on Friday from injuries sustained during enemy action in the same district.
