Twelve civilians were killed and 18 were wounded when NATO-led forces carried out an airstrike on suspected Taliban militants in Kapisa province on Saturday, according to provincial officials.

President Hamid Karzai has ordered an investigation.

The issue of civilian casualties has led to a rift between Afghanistan and its Western allies with President Hamid Karzai saying on Sunday that foreign airstrikes had only succeeded in killing ordinary Afghans and would not defeat the insurgency.

The NATO-led International Security Assistance Force (ISAF) says it has conducted an investigation and maintains those killed were insurgents.

"Following recent media reports of significant civilian casualties ... ISAF has now concluded a thorough investigation of the claim and has evidence that those killed were insurgents," ISAF said in a statement late on Monday.

Insurgents fired on ISAF soldiers in Tagab district in Kapisa province on Saturday but withdrew after two of their soldiers were wounded, ISAF said.

A surveillance aircraft later observed the men hiding their weapons and changing into civilian clothes, it said.

"Still under observation, they then moved to an open field where they were attacked and killed by shots from an ISAF aircraft," ISAF said. "No innocent civilians were injured or killed."

About 1,000 civilians have been killed during the first six months of this year, aid agencies say. While most are believed to have died during insurgent attacks, many Afghans feel foreign forces don't take enough care in avoiding civilian casualties.

Violence has escalated in Afghanistan since 2006 when the Taliban, ousted from power in 2001, relaunched their insurgency against the government and foreign forces.

The defence ministry on Tuesday said six insurgents were killed in an operation in the southeastern province of Paktika.

Elsewhere, an improvised explosive device killed a Latvian soldier and wounded two more on Monday, ISAF said in a separate statement late on Monday. Thirteen civilians were also wounded in the attack, it said.