Four soldiers belonging to the NATO-led force in Afghanistan have been killed by insurgents during an operation, while a fifth was shot dead by a security guard over the weekend in the country's south.

The Taliban have been recovering ground in the south as forward operating bases have shut as part of the plan to withdraw most foreign troops by the end of 2014.

The nationalities of the dead soldiers were not released.

Also in southern Afghanistan, a contracted security guard shot one soldier dead in a separate incident yesterday.

A spokesman for the NATO-led force said the nationality of the security guard was under investigation.

So-called "insider attacks" on foreign soldiers by their Afghan allies escalated last year prompting coalition troops to heighten security measures and reduce interaction between foreign and Afghan forces.

The move has helped stem the number of incidents this year but also further eroded trust between foreign service members and their Afghan allies.

If yesterday's gunman turns out to have been an Afghan security guard for coalition troops, it would be the ninth insider attack this year bring the toll of foreign personnel killed to 14.

Meanwhile, Afghan President Hamid Karzai criticised his NATO allies today over an air strike in the country's east that he said killed five civilians, including three children.

The strike on Friday followed a mortar attack on a joint NATO-Afghan base near a village just outside Jalalabad city, NATO and local officials said.

Civilian casualties are straining the relationship between Karzai and his international backers just as Washington tries to finalise a vital security pact with Kabul.

"President Hamid Karzai strongly condemned the NATO airstrike in which he says five civilians, including three students aged 10, 14 and 16, were killed in eastern Nangarhar province on Friday night," a statement from Mr Karzai's palace said.

The three students were brothers, the statement said.

NATO's International Security Assistance Force initially denied civilians had been involved in the attack, but later launched an investigation into the incident.

"ISAF takes all allegations of civilian casualties seriously and is working with our Afghan partners to confirm the details, "a NATO spokesman said yesterday.

Civilian casualties have been on the rise in the Afghan war, adding to concern about security as Western forces prepare to leave by the end of next year.

The strike comes less than a month after NATO was forced into a similar investigation when a drone strike targeting senior al Qaeda and Taliban leaders hit a truck and killed as many as eight women and children.

NATO at first said they had killed ten "enemy forces", but launched an investigation when images of dead civilians were shown to them. That investigation is ongoing.

The latest strike comes as Washington and Kabul try to finalise a vexed and much-delayed security pact that will shape the post-2014 international military presence here.

The negotiations have stalled over, among other things, Afghan concerns regarding a U.S. desire to undertake counter-terrorism operations in the country after next year.