The White House expressed "great concern" on Friday over the loss of civilian life following reports that a NATO air strike in Afghanistan killed as many as 90 people.
White House spokesman Robert Gibbs said the incident, which occurred earlier on Friday when NATO warplanes struck two fuel trucks that had been hijacked by the Taliban, would be investigated.
"Obviously, any time there is loss of life in a conflict like this, particularly the civilian loss of life, it's something we've expressed in the past and continue to express great concern about," he said.
Gibbs also highlighted that US ambassador in Kabul, Karl Eikenberry, had been involved in previous such investigations and offered US condolences to the families of those who lost their lives.
Officials said the NATO bombing killed 56 to 90 people in Afghanistan, when the NATO air strike blew up two fuel tankers hijacked in northern Kunduz by the Taliban en route from Tajikistan to Kabul.
They said the dead were mostly insurgents, but Afghan President Hamid Karzai said any targeting of civilians was unacceptable.
Karzai's office said 90 people were killed and hurt. Police and the interior ministry put the toll at 56 Taliban killed and 10 more wounded.
Witness Mohammad Daud, 32, said villagers rushed to one of the trucks when it got stuck in the river to take free fuel at the Taliban's invitation.
"Everyone around the fuel tanker died. Nobody was in one piece. Hands, legs and body parts were scattered everywhere. Those who were away from the fuel tanker were badly burnt," he told AFP.
Pentagon spokesman Geoff Morrell on Friday would not comment on the latest incident, saying the investigation was still ongoing.
But he added that it had never been assumed that a recent directive by US and NATO commander in Afghanistan General Stanley McChrystal restricting the use of air strikes would "eliminate all civilian casualties in the future."
Instead, the move aimed to "try to foster and build a closer relationship between our forces and the Afghan people so that they trust that we are trying to minimize civilian casualties, that we are trying to help them."