Afghan authorities said Thursday they were investigating reports of civilian deaths in a NATO air strike, as fresh protests erupted over the alleged killing of 10 non-combatants.

The probe was launched into reports that nine civilians were killed in a NATO air strike near the town of Lashkar Gah in the troubled southern province of Helmand on Wednesday.

Sensitivities about civilian casualties allegedly caused by NATO or US-led operations have driven a wedge between Afghan President Hamid Karzai and his backers in the international military.

The United States and NATO have 113,000 troops fighting Taliban-linked militants in a war now into a ninth year and becoming increasingly lethal, for civilians as well as combatants.

Karzai has said that members of foreign forces shot dead unarmed civilians, including eight teenagers, in the northeastern province of Kunar on Saturday.

NATO's International Security Assistance Force (ISAF) denied the dead were unarmed and has said that no NATO or US military personnel were involved in the incident.

A senior military official said on condition of anonymity that the deaths occurred in a "sanctioned operation" by US non-military personnel who shot back in self-defence.

Around 1,500 people took to the streets for the latest in a string of protests over the alleged killings, using sticks to beat an effigy of US President Barack Obama and shouting "death to Obama", witnesses said.

Hundreds of students led the peaceful march through Asadabad, capital of Kunar, chanting "death to America," "death to Britain and those who killed the students", witnesses said.

"We want the perpetrators brought to justice. The coalition forces must stop unilateral operations and all joint operations must be conducted during the day, rather than at night so their targets are clear," said organiser Abdul Wahab.

Daud Ahmadi, a spokesman for Helmand governor Gulab Mangal, told AFP that civilians had been killed in the Lashkar Gar bombing, but he had no figures or other details.

"We know civilians have been killed but we don't know how many. The governor has sent a delegation to the area to provide some cash support to the victims' families and investigate the incident," Ahmadi said.

Mohammad Alam, who said he had taken a wounded man to the city's hospital, said villagers had gathered to discuss water distribution when the air strike took place late Wednesday.

"We'd gathered in the village to discuss water. All of a sudden the area was bombed, eight people were killed on the spot, another was wounded whom I brought to hospital, he died later," he said.

Ahmadi said the air strike was launched after troops came under fire from the same village.

The war of words over civilian casualties came as the Taliban claimed responsibility for a suicide attack on a US base that killed eight US civilians, and after a bomb attack killed five Canadians, including a reporter.