KABUL: Afghan President Hamid Karzai criticised his Nato allies on Sunday 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 air strike 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 Karzai's palace said.

The three students were brothers, the statement said.

Nato's International Security Assistance Force (Isaf) 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 on Saturday.

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 US desire to undertake counter-terrorism operations in the country after next year.