Angry Afghans shouted "death to America" and hurled stones at government offices Thursday to protest against the killing of civilians in US-led air strikes, witnesses said.

Shots were also fired into the air during the protest, although it was not immediately clear by whom, while one demonstrator said the protest in the western town of Farah was gaining strength and threatened to turn violent.

US military and Afghan teams pressed an investigation Thursday into how many civilians and insurgents were killed in fighting and air strikes earlier this week in what appeared to be one the deadliest incidents in eight years.

Afghan police said more than 100 people were dead, most of them civilians. One Afghan official said he had seen the bodies of 20 children.

"The protest is getting bigger and bigger," a demonstrator who gave his name only as Abdullah told AFP. "People are really angry and they shout 'death to America, death to the invaders.'

"They are hurling stones at government buildings and there is some gun fire in the air -- at this stage we don't know by who."

Haji Nangyalai, 42, said the protest was to "show our anger at the crimes committed by the American forces. They have killed our innocent people carelessly, that is why we are protesting."

Police were monitoring the protest, provincial police chief Abdul Ghafar Watandar told AFP.

A delegation of foreign troops and Afghan government officials left for Bala Buluk, about 50 kilometres (30 miles) north of the provincial capital, earlier in the day to continue investigations, he said.

The police spokesman for western Afghanistan, Abdul Rauf Ahmadi, told AFP Wednesday that according to information passed to him more than 100 people died in the air strikes and ground operations.

Twenty-five to 30 were Taliban and the remainder were civilians including children, women and elderly people, he said.

The killing of civilians in the fight against extremists in Afghanistan is a main source of tension between Kabul and Washington, on which the fragile country depends for security and aid.

The United States has around 38,000 troops in Afghanistan, the bulk of a foreign deployment of roughly 70,000 tasked with hunting down armed extremists and stabilising the country.