Kabul - US Defence Secretary Roberts Gates apologized for civilian casualties caused by his country troops in Afghanistan on Thursday, but blamed the Taliban for using civilians as human shields.

Gates' comments came as Afghan and US military officials were investigating allegations that more than 100 civilians were killed in a US military airstrike in Bala Boluk district of western Farah province.

"We regret any, even one, Afghan civilian casualty ... innocent civilian casualty," Gates told a press conference in Kabul on Thursday.

"I believe in many instances the Taliban use civilians as shield," he said, adding that the Afghan people "need to recognize that exploiting civilian casualties and causing civilian casualties are parts of fundamental Taliban strategy and it is measure of ruthlessness with which they fight."

The US defence chief said that mounting civilian deaths could "pose a risk" to efforts of international military forces that have been trying to bring peace and reconstruction to the war-torn country.

"What is critical for the success of the Afghan government and for us as the governments and Afghan people's partners is that the Afghan people believe that we are in their sides, that we are here to help them," he said, adding, "we are here to protect them not to hurt them."

On Thursday morning, hundreds of people gathered in the main mosque in Farah, the capital of the province of the same name, chanting slogans against the US forces in the country and Afghan government.

The protesters claimed that more than 100 civilians lost their lives in the air raid in the district. If confirmed, it would be the deadliest incident involving civilian fatalities since the ouster of the Taliban regime in late 2001.

The International Committee of the Red Cross said Tuesday their team saw dozens of bodies at the attack site.

Afghan security forces fired shots into the air when the protestors tried to approach the provincial governor's office, Ahmad Wali, a Farah resident, told the German Press Agency dpa.

He said several people were wounded when the demonstrators hurled stones at security forces and they responded with gunshots.

"Three people were wounded during the demonstration and receive treatment in a city hospital," Farah legislator Mohammad Musa Nasrat said, adding that he was not certain if they were wounded by bullets, or trampled by other protestors when they fled the area after the shots were fired.

"More than 1,000 people took part in today's protest, and they just want their voice to be heard and the culprits be punished," Nasrat said. "The residents in Farah city closed their shops and gathered in the central mosque to pray for the dead civilians and express their anger against this barbaric act by American forces."

A joint team of Afghan government officials and US military forces were scheduled to visit the concerned villages Thursday to investigate the incident, Deputy Provincial Governor Mohammad Younus Rasouli told dpa.

The airstrikes hit the area Monday night after militants killed three civilians and three policemen in Girani village. Afghan officials said that at least 25 militants were also killed in the operation.

Civilian casualties caused by US strikes have been an ongoing source of tension between the US and Afghan leaders.

More than 2,100 civilians were among thousands of people - mostly insurgents - killed in Afghanistan's conflict last year, according to the United Nations. Afghan and UN investigators said that around 90 civilians were killed in a US air raid in the western province of Herat last August.

The US military initially denied many civilians had been killed in that incident, but later acknowledged that 33 people were killed when videos showing dead bodies of women and children emerged.

Gates's visit comes months before more than 20,000 additional US are expected to arrive in Afghanistan. Currently around 70,000 international forces, with more than half of them American soldiers, are stationed in the country.