A US military spokeswoman confirmed the attack, but said the men were insurgents spotted loading ammunition on a truck. She said the incident was under investigation.

Civilian deaths have long been a source of tension between the government of Hamid Karzai and NATO forces. Karzai has repeatedly warned the alliance of failure in the fight against terrorism if the killing of civilians continued.

NATO's new secretary general, Anders Fogh Rasmussen, who came to Kabul on a two-day visit Wednesday, told a press conference that the alliance forces would do their 'utmost to reduce the number of civilian casualties to an absolute minimum.'

'We cannot accept the loss of innocent life,' Rasmussen said, adding: 'Unfortunately in an armed struggle we will see civilian casualties.'

Meanwhile, five Afghan police were killed and three others wounded when their vehicle struck a roadside bomb in Nad Ali district of Helmand province Thursday morning, Helmand police chief Sherzad said.

In another incident, a US soldier was killed by a roadside bomb explosion in western Afghanistan Wednesday, the NATO said in a statement.

More than 100,000 international troops deployed from 42 nations are currently stationed in Afghanistan. Over 60,000 of that number are US soldiers.