An Afghan soldier shot dead two US soldiers and one US civilian on Saturday, the latest "insider attack" to shake efforts by the two armies to work together to defeat the Taliban insurgency.
The killings in the eastern province of Paktika came on the same day that one Italian soldier died when a grenade was thrown into an armoured vehicle in Farah province, in the far west of the country.
Scores of foreign soldiers have been killed in insider attacks in Afghanistan, breeding fierce mistrust and threatening to derail the training of local forces to take over security duties ahead of NATO's withdrawal next year.
The threat has become so serious that foreign soldiers working with Afghan forces are regularly watched over by so-called 'guardian angel' troops to provide protection from their supposed allies.
The Paktika provincial government issued a statement saying "a verbal dispute erupted" between the Afghan soldier and a US soldier in the district of Khair Kot.
It said the Afghan man killed three US service members from the NATO-led International Security Assistance Force (ISAF) and wounded three others before being shot dead himself.
"Initial information indicates that the incident followed an argument, and the soldier was not connected with the armed opposition," the statement added.
ISAF officials say that most insider attacks stem from personal grudges and cultural misunderstandings rather than Taliban insurgent plots.
An ISAF statement confirmed the incident, saying that two US soldiers and one US civilian had died. It gave no further details on the role of the civilian.
Afghan soldiers and police are taking on responsibility for battling the militants from 100,000 NATO combat troops who will leave by the end of 2014 -- 13 years after a US-led invasion brought down the Taliban regime.
In Farah province, the Italian soldier was killed and three others were wounded when a grenade was thrown into their vehicle, officials said.
"Today at around 10:30 am, an enemy on a motorcycle threw a grenade on a convoy of Italian forces who were driving on a patrol in Farah city," Abdul Rahman Zhowandi, Farah provincial spokesman, told AFP.
The Italian defence ministry said the attack happened as the convoy was returning to base.
So far this year, 85 ISAF soldiers have died in Afghanistan, according to the independent icasualties website. In 2012, 402 died, down from a peak of 711 in 2010.
Casualty rates in the Afghan security forces have risen by 15-20 percent in the last year or two, officials say, though exact figures are not released.
On Saturday, the government said its forces had retaken one of the five districts still ruled by the Taliban.
Soldiers and police moved into the centre of Waygal district in the mountainous eastern province of Nuristan overnight after two days of fighting, interior ministry spokesman Sediq Sediqqi told reporters in Kabul.
He vowed that the remaining insurgent strongholds, in Helmand, Ghazni and Zabul provinces, would be won back before presidential elections next April.
However the insurgents still wield influence over much of the rest of the country, and fears are growing that the national security forces will be unable to provide security after the departure of NATO troops.