"There were civilian casualties no doubt," he said on Friday after US military and Afghan teams returned from a joint investigation in Farah.

"But the conclusion from the investigation has not been reached, and it's inappropriate to indicate one way or the other how they were caused."

'Civilians as cover'

Julian said earlier that the Taliban had used civilian positions as cover for attacks on US and coalition troops.

"[Civilian casualties] are always a possibility when conducting counterinsurgency activity," he said.

"We are here to protect the civilian population and we take this very seriously. The last thing we want to have happen is innocent civilians to be injured or killed."

During a visit to Kabul on Thursday, Robert Gates, the US secretary of defence, said he had heard reports that the Taliban threw grenades into houses during the clashes to create casualties that could be blamed on American forces.

Hundreds of Afghans gathered to denounce the killings, demanding that US troops leave Afghanistan.

The US government has come under increasing criticism during the past year for civilian deaths during operations against Taliban and al-Qaeda fighters in Afghanistan.

Barack Obama, the US president, and other Washington officials have repeatedly expressed regret for civilian casualties and vowed to take measures to avoid them in the future.