The shooting in Paktika province was the latest in a string of so-called "insider attacks" in which Afghan forces open fire on their own comrades or international troops. The incidents threaten to shake the confidence and trust of the two sides as the 2014 withdrawal of most of the international forces approaches.
An argument between the Afghan soldier and his trainers appeared to have led to Saturday's shooting on an Afghan National Army base in Paktika's Kher Qot district, according to a statement from the provincial governor's office. The international military coalition in Afghanistan said two American service members and one U.S. civilian died. It had initially identified them as three U.S. military personnel.
The angry Afghan soldier opened fire during the argument, killing the three foreign trainers and wounding three others, according to the governor's statement. The foreigners returned fire and killed the Afghan soldier, who had no known connection to the insurgency.
A second Afghan man was arrested after the shooting, and an investigation has been launched, the coalition said later Saturday.
So far this year, there have been five insider attacks on foreign forces, with a total of eight troops and one U.S. contractor killed. However, the number of such attacks has eased after soaring last year — in 2012, there were at least 29 insider attacks, killing 62 international troops.
Afghan security forces also are targets of such attacks. Last month, two recently rehired Afghan police opened fire on their commander at a checkpoint in a remote district in the country's south, killing him and six of his men.
The Taliban insurgents claim most of the insider attacks, saying they have infiltrated Afghan security forces or persuaded soldiers and police to join their side. However, the international coalition has said many of them are sparked by personal disputes.
In the western province of Farah, meanwhile, an Italian soldier was killed and three others wounded in a grenade attack on their armored vehicle in western Afghanistan.
The Italian Defense Ministry said the attack in Farah province came as the Italian soldiers were returning to their base from training Afghan security forces.
The Italian convoy of three armored vehicle apparently had been slowed by traffic near an intersection when an attacker ran up and threw an explosive device into the lead vehicle, the ministry said. It added that the three wounded soldiers' injuries were not life-threatening.
The Taliban quickly took responsibility for the attack, with spokesman Qari Yousef Ahmadi claiming that the attacker was an 11-year-old boy.
But Farah province provincial government spokesman Abdul Rahman Zhawandai says an adult man was seen throwing a grenade, then escaping by blending into the crowd at a nearby vegetable market.
Saturday's deaths brought to 15 the number of international troops killed in Afghanistan this month. On Thursday, seven Georgian soldiers died in a truck bombing at their base in the south.
Taliban insurgents have launched intense attacks across the country as Afghan forces take over most security responsibility ahead of most foreign troops' withdrawal next year, more than a decade after the American-led invasion to oust the Taliban regime for sheltering al-Qaida's leadership after the Islamic extremist group launched the Sept. 11, 2001 terrorist attacks in the U.S.
Countries in the NATO alliance met in Brussels this week to lay out a new plan for shifting into a training and assistance role, but they did not agree on how many noncombat troops it will maintain in Afghanistan after 2014.
There are now about 100,000 international troops in Afghanistan, including 66,000 from the United States. Most of the pullout is set for this winter.
The remaining smaller force is expected to be mostly American advisers. However, German Foreign Minister Guido Westerwelle said Saturday during a surprise visit to Kabul that the German government is thinking about leaving behind 600-800 troops after 2014.
"The departure of our troops from Afghanistan will happen as planned but at the same time we will not forget about Afghanistan in the years after 2014," Westerwelle said, adding, "We will engage with them in a different way."
Rahim Faiez in Kabul and Frances D'Emilio in Rome contributed to this report