Media playback is unsupported on your device Media caption Brigadier General Gunter Katz said Isaf was conducting investigations into the killings

Two US soldiers and an American civilian have been killed in a possible insider attack in eastern Afghanistan, the Nato-led force says.

A man wearing Afghan army uniform was said to have opened fire on them in Paktika province, close to the border with Pakistan,

Isaf forces have taken a series of measures to try to halt such attacks.

An Italian soldier died in a separate attack in Farah province in the west of the country, Italian officials said.

There was a spate of insider attacks on foreign troops last year but the last was more than a month ago, the BBC's David Loyn reports from Kabul.

Coalition forces train, mentor and fight alongside Afghan soldiers and that makes them vulnerable, our correspondent says.

"An individual wearing ANA (Afghan National Army) uniform turned his weapon against the Isaf service members," an Isaf statement said.

Three other Americans were wounded in the shooting, which appeared to start after an argument, Paktika's governor said. Isaf troops returned fire, killing the Afghan soldier, he added.

"Today was a very difficult day for Isaf," said spokesman Brig Gen Guenter Katz. He said they were conducting investigations, but it was too soon to give more details.

'Insurgent attack'

Meanwhile, the Nato-led coalition said the soldier killed in western Afghanistan had died "during an attack by an enemy of Afghanistan".

It was an "insurgent attack", Isaf said, rather than an insider incident as initially thought.

One soldier was killed by "hostile elements" and three more were wounded, the Italian defence ministry said.

A Lynx vehicle was attacked at 10:30 local time (05:00 GMT) as it returned to base as part of a Military Adviser Team convoy after training Afghan security forces, a statement said.

Details of the attack are still being investigated but initial reports say a grenade was thrown at the vehicle by a man wearing an Afghan army uniform, Ansa news agency says.

The Taliban later said an 11-year-old boy had carried out the attack.

The dead soldier was later named as Giuseppe La Rosa, a 31-year-old member of the Third Bersaglieri Regiment.

Last September, Nato announced it was limiting joint patrols with Afghan forces. Only large operations would be conducted jointly, with joint patrols evaluated on a case-by-case basis, Nato said.

Isaf forces have also been using "guardian angels" - Nato soldiers who provide a round-the-clock armed guard for their colleagues while they are training their Afghan counterparts.

About 7,000 new recruits a month join the Afghan army alone. Correspondents say it is a huge challenge to ensure Taliban militants do not slip through the net.