A man in an Afghan uniform turned his weapon on American trainers working with him in the eastern province of Paktika, killing three of them, while an attacker with a grenade killed an Italian soldier in the west, officials have said.

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.

There was no immediate claim of responsibility, but NATO officials have said that most insider attacks stem from personal grudges and cultural misunderstandings rather than Taliban plots.

The international military coalition in Afghanistan said two American service members and one US civilian died. It had initially identified them as three US military personnel.

The foreigners returned fire and killed the Afghan soldier, who had no known connection to the insurgency, according to the local governor's statement.

The deaths of the International Security Assistance Force on Saturday in the district of Khair Kot came on the same day that one Italian soldier was killed and three others were wounded when a grenade was thrown into their armoured vehicle.

An Italian soldier was killed and three were wounded in Farah city on Saturday morning when an 11-year old child threw a grenade at a NATO convoy in the western province of Farah, a Taliban spokesman said.

'Guardian angel' troops

The Italian defence ministry and military said the attack happened as the convoy was returning to base.

The four deaths on Saturday bring to 16 the number of international troops killed in Afghanistan this month, and come two days after seven Georgian soldiers were killed in a suicide car-bomb attack in the southern province of Helmand.

Scores of foreign soldiers have been killed in insider attacks, breeding fierce mistrust and threatening to derail the process of training Afghan 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 Taliban launched their annual spring offensive at the end of April, marking a crucial period for Afghanistan as local security forces take the lead in offensives against the insurgents.

The militants said multiple suicide bombings and insider attacks by Afghan soldiers on NATO-led troops would be used to inflict maximum casualties.

There are now about 100,000 international troops in Afghanistan, including 66,000 from the United States. Most of them are due to leave this winter.

The remaining smaller force is expected to be mostly American advisers.

However, Guido Westerwelle, the German Foreign Minister said on Saturday during a surprise visit to Kabul that the German government was 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.

"We will engage with them in a different way.''