A US soldier was killed in the latest in a series of "green-on-blue" attacks by Afghan allies in a bloody day across the war-torn country on Tuesday.
The day began with a pre-dawn remote-controlled explosion which killed nine civilians in a minibus on the outskirts of the capital Kabul, followed hours later by a suicide truck bomb attack on a NATO military base.
A French soldier and around 10 Taliban fighters were also killed in an early morning ambush and subsequent firefight during a joint operation with the Afghan army in Kapisa province near Kabul, French officials said.
But in a country where roadside bombs, firefights and suicide attacks are commonplace, it is the death of the US solider at the hands of men he was working alongside that will resonate most deeply with Western forces.
"An International Security Assistance Force service member died when two individuals wearing Afghan National Army uniforms turned their weapons against ISAF service members in eastern Afghanistan today," ISAF said.
A US defence official confirmed the soldier was an American, and said the two suspected shooters were in custody. An investigation was under way to determine whether they were Taliban infiltrators.
The shooting is the latest in an increasing number of attacks in which Afghan soldiers have turned their weapons against NATO troops helping Kabul fight a decade-long insurgency by hardline Taliban Islamists.
The death takes the green-on-blue toll this year to at least 30, in 21 such incidents, according to an AFP tally.
Some of the attacks are claimed by the Taliban, who say they have infiltrated Afghan army ranks, but many are attributed to cultural differences and antagonism between local and US-led allied forces.
The powerful truck bomb exploded at a NATO military base some 70 kilometres (40 miles) south of the capital Kabul, in Logar province, amid growing unrest in areas neighbouring the Afghan capital.
Afghan police said at least 17 civilians and three soldiers were wounded.
ISAF acknowledged that some soldiers were injured in a blast at the base, but gave no figures.
"I was on my way to school when there was suddenly a huge explosion which knocked me down," schoolboy Samiullah told an AFP reporter at the scene.
"I saw thick smoke and flames rising from the inside of this camp."
Taliban insurgents claimed responsibility for the attack.
Just hours earlier, a Taliban bomb killed nine people and wounded five when it struck a minibus on the western outskirts of Kabul.
President Hamid Karzai condemned the attack, saying: "Terrorists who plant roadside bombs on public routes during the holy month of Ramadan, targeting and killing innocent Muslim civilians, are definitely neither Muslims nor Afghans."
The series of assaults will add to growing concerns over the country's future once some 130,000 NATO troops withdraw as planned by the end of 2014, handing responsibility for security to Afghan forces.
Western politicians keen to get their troops out of an increasingly unpopular war regularly talk up the ability of the Afghan army and police to cope on their own, but there is widespread fear of a multi-factional civil war once they leave.