The three Americans were shot dead by the man following an argument, the Paktika provincial governor's spokesman Mukhlas Afghan said, adding that three other Americans had been wounded.

The attacker was himself shot dead soon after opening fire, the spokesman said.

A previous statement had said all three were soldiers.

Also on Saturday, an Italian soldier was killed and three were wounded when a child threw a grenade at a NATO convoy in the western province of Farah, a spokesman for the governor and a Taliban spokesman said.

So-called insider attacks by Afghan soldiers on their NATO-force allies have become an increasing problem over the past year or so, threatening to undermine already waning support for the war in Western nations sending troops.

Last year, a surge in such attacks prompted NATO to temporarily curtail some joint operations with Afghan government forces.

The Paktika provincial governor's spokesman, Mukhlas Afghan, said an Afghan soldier had killed three American soldiers. Afghanistan's NATO-led forces said three of its soldiers had been killed by a man in an Afghan army uniform but did not give nationalities.

Insider attacks accounted for one in every five combat deaths suffered by NATO-led forces in Afghanistan and 16 percent of all American combat casualties, according to 2012 data. 

The toll has alarmed Afghanistan's Western allies and raised troubling questions about the unpopular war's direction as most international forces prepare to withdraw by the end of next year. 

"A brave, heroic 11-year-old Afghan child hurled a hand grenade at dismounted Italian troops in Farah city," the Taliban said in an English-language statement. 

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.