An Afghan national working on an installation shared by Afghan and foreign forces has shot dead three US service members, raising to six the number of Americans killed by their Afghan partners in a single day, officials said.
The newly announced killings took place on Friday, the same day that an Afghan policeman gunned down three US Marines in a separate attack in southern Afghanistan.
Such assaults are on the rise and have heightened mistrust between foreign forces and the Afghan soldiers, police and others they are training and mentoring.
Four of the attacks occurred in the past week, raising questions about the safety of international trainers more than 10 years into the war.
The US-led coalition insists the attacks do not represent the overall security situation in Afghanistan and that they have not impeded ongoing work to hand over security to Afghan forces by the end of 2014.
Most of the attacks have been carried out by Afghan police and soldiers or militants wearing their uniforms. There have been 26 such attacks so far this year, resulting in 34 deaths, according to the US-led coalition.
The Taliban claimed responsibility for both attacks in Helmand province - an area of the south where insurgents have wielded their greatest influence.
In the first attack, an Afghan police officer shot and killed three Marines after sharing a pre-dawn meal with them in the volatile Sangin district, according to Afghan officials. A US Defence Department official confirmed that the dead Americans were Marine Special Operations Forces.
Sangin's district chief and the Taliban both identified the gunman as Asadullah, a member of the Afghan National Police who was helping the Marines train the Afghan Local Police, a village-level defence force overseen by the Ministry of Interior. The district chief, Mohammad Sharif, said the shooting happened at a police checkpoint after a joint meal and a security meeting. The meal took place before dawn because of Ramadan, the month in which Muslims abstain from food during daylight hours.
Sidiq Sidiqi, a spokesman for the Afghan Ministry of Interior, told reporters the shooter may have been a member of the Afghan Local Police, rather than the National Police. He said investigators were still reviewing the case.