Afghanistan's President Hamid Karzai has accused NATO of killing a number of children in an airstrike, a case which could stoke tensions between the government and its western backers over a mounting civilian death toll.

The NATO-led coalition in the country did not immediately confirm the deaths, but said it was investigating an incident in the Najrab district of eastern Kapisa province.

Civilian deaths have been one of the biggest sources of friction in relations between the Afghan government and NATO ahead of a withdrawal of foreign combat troops in 2014.

"President Hamid Karzai strongly condemned an airstrike by foreign troops which resulted in the killing of a number of children," a statement from his office said.

Karzai had sent an advisor, Mohammad Zahir Safi, to the area to investigate the incident, the statement added.

Mehrabuddin Safi, the governor of Kapisa, said a coalition air strike late on Wednesday killed eight children in Giawa village. Other Afghan officials had earlier said the strike followed a night raid on suspected insurgents.

"The matter is currently being assessed by a joint assessment team to determine the facts," a spokesman for the 130,000-strong International Security Assistance Force in the country said.

A United Nations report last week said the number of civilians killed and injured in the Afghan war had risen for the fifth year in a row, lifting from 2,790 in 2010 to 3,021 civilian deaths in 2011.

Most deaths were caused by insurgents, the report said, but civilian deaths due to NATO air strikes also rose nine percent to 187, and were the coalition's biggest civilian killer.