The US says that it is determined to reduce civilian casualties
The US military has opened a criminal investigation into allegations that American soldiers were involved in the unlawful deaths of Afghan civilians.
A statement released by the US Army in Afghanistan says that a small number of US soldiers were responsible.
It said that "as many as three Afghan civilians" were killed.
A spokesman for the US military in Kabul told the BBC's Mark Dummett that there had been "no other similar cases as serious as this one".
The spokesman said that he could not give more information because he did not want to jeopardise the investigation.
The statement released by the military said that there were also allegations of illegal drug use, assault and conspiracy. So far no charges have been made.
However the statement says that one soldier has been placed in pre-trial confinement.
Source of tension
"The army's Criminal Investigation Command initiated their investigation after receiving credible information from the soldiers' unit earlier this month," the statement said.
Civilian causalties have caused deep unrest in Afghanistan
"We are committed to the security and safety of the Afghan population and will ensure any crimes are investigated fully and those responsible will be held accountable."
Correspondents say that the military did not detail when, where or under what circumstances the Afghan civilians died.
However military spokesman Lt Col Joseph Breasseale told the AFP news agency that "the investigation is in Afghanistan, not in the US, and the soldiers are currently in Afghanistan".
Civilian casualties in the nine-year conflict in Afghanistan are deeply controversial and have been a constant source of tension between Afghan President Hamid Karzai and US and Nato forces.
President Karzai has repeatedly urged foreign forces to take "serious precautions" to avoid civilian casualties.
In a joint news conference with President Karzai last week, US President Barack Obama said that US forces would strive to minimise harm to civilians in their operations and that he was "accountable" for civilian deaths that occurred.
In February the commander of international forces in Afghanistan, Gen Stanley McChrystal, apologised to President Karzai for a Nato air strike in Uruzgan province which killed at least 27 civilians.
Gen McChrystal has also pledged to reduce civilian casualties.
He has introduced changes to Nato tactics aimed at cutting the risks to civilians. Measures include reducing the number of air strikes and night raids.