Image copyright Reuters Image caption All Nato combat forces are due to leave Afghanistan by the end of the year

Five American soldiers have been killed accidentally by their own side in southern Afghanistan, US military sources have confirmed.

An Afghan soldier and an interpreter also died in the Nato air strike after Monday's operation in Zabul province

Afghan officials say coalition forces called in air support when they were attacked by the Taliban at the end of the operation. Nato is investigating.

Militants have stepped up attacks as foreign combat troops leave this year.

The incident, which happened on Monday, is among the most serious cases of so-called "friendly fire" in Afghanistan, where Nato-led troops have been battling Taliban and other insurgents since 2001.

A statement from international forces in Kabul said: "Tragically there is the possibility that fratricide may have been involved."

Nato did not immediately confirm the nationality of these latest casualties.

But the BBC's David Loyn in Kabul says the term "fratricide" is used by US forces when they mistakenly kill soldiers on their own side.

Analysis - Military historian Peter Caddick-Adams

Chroniclers of the battle of Waterloo, fought in 1815, have recorded how British infantry squares engaged one another by mistake and other allied cavalry, causing many casualties.

Similar incidents happened in the Crimea in 1854, during the American Civil War of the 1860s and the Boer War of 1899-1902.

War diaries from World War I are peppered with accounts, mainly of British artillery shelling British troops by accident, poison gas clouds being misdirected, or a worn gun barrel firing shells inaccurately.

In World War Two, many allied aircraft were lost to so-called friendly fire, because of poor aircraft recognition skills, or the split seconds in which a pilot had to decide whether to engage an oncoming plane or not.

Historians now think that the famous RAF fighter ace Douglas Bader was shot down by one of his wingmen, not the Germans.

A history of friendly fire

The Isaf force currently has soldiers from 50 contributing nations in Afghanistan. Most troops stationed in the south are American.

The governor of Zabul province told the BBC there was a joint operation involving Afghan and international troops.

The incident happened in Arghandab district, a place hotly contested between the Taliban and international forces for some years, our correspondent reports.

Southern Afghanistan is the heartland of the Taliban movement and insurgents frequently attack security forces in the region.

There have been more than 30 Nato forces killed this year in Afghanistan - the latest incident is the deadliest so far in 2014.

Afghanistan is set to hold a run-off round of voting in its presidential election on Saturday.

Insurgents - who vowed to disrupt campaigning and voting - attacked the campaign rally of one of the contenders last week.