Hundreds of Afghan protesters blocked highways in two separate demonstrations on Tuesday, accusing US-led NATO troops of killing children and civilians, officials and witnesses said.

In the first demonstration, protesters carrying the bodies of four children aged eight to 12 blocked the Kabul-Kandahar highway in the south and chanted anti-US slogans, they said.

A spokesman for NATO's International Security Assistance Force (ISAF), Lieutenant Commander Brian Badura, said initial reports suggested that the children were killed by insurgent fire.

"We are aware of an incident in southern Afghanistan yesterday. ALP (Afghan Local Police), along with coalition advisors, were meeting with local villagers when they were attacked by insurgents," he said.

"Initial reporting indicates there were a number of children killed and wounded by the insurgents. The children were evacuated by coalition forces to a medical facility for treatment."

The deputy governor of Zabul province, Mohammad Jan Rasoulyar, said the children died during an exchange of fire between Taliban insurgents and ISAF and Afghan security forces in Shahjoy in the troubled southern province on Monday.

The gunfight began when the security forces were meeting local community leaders and came under attack by insurgents.

"ISAF and Afghan troops returned fire. There was an exchange of fire during which four children were killed and some others kids were injured. Today the people are protesting the killing of the children," Rasoulyar told AFP.

The official said his initial findings suggested the casualties were caused by Taliban fire.

Police official Mohammad Zahir told AFP that "a few hundred" people took part in the protest. The men carried the bodies of four children they alleged were killed in ISAF fire, he said.

Civilian casualties are a sensitive issue in the US-led war against the Taliban insurgency and have often been the cause of tense relations between Kabul and Washington.

In a separate demonstration in eastern Laghman province, protesters carrying the bodies of two men blocked the highway from Kabul to Jalalabad, chanting "Death to America and Death to (Afghan President Hamid) Karzai".

Mohammad Aziz Kuchi, the uncle of one of the dead men, said that "around 2:00am last night, Afghan and foreign forces entered our house. They opened fire on us, killing two young men.

"They took eight others away, including a 75 year-old man. They were all civilians, with no links to the Taliban."

ISAF said in a statement: "In Qarghahi district, Laghman province, an Afghan-led and coalition supported security force conducted an operation to detain a Taliban leader today.

"The leader coordinated roadside bombings against Afghan and coalition security forces throughout the province.

"During the operation, the leader and one additional insurgent fired on the security force. The force returned fire, killing the attackers.

"The security force also detained multiple additional insurgents and confiscated several small arms weapons."