The tankers were crossing a river when the air strike took place
Scores of people are reported killed after a Nato air strike blew up two fuel tankers hijacked by the Taliban in northern Afghanistan.
The governor of Kunduz province told the BBC that Taliban leaders were among at least 90 killed. Witnesses said locals taking fuel also died.
Nato said its commanders believed only insurgents were present but that it had reports many civilians were injured.
Secretary General Anders Fogh Rasmussen said an investigation was under way.
A statement from Hamid Karzai's office said the Afghan president believed that "targeting civilians in any form is unacceptable and [he] emphasised that innocent civilians must not be killed or wounded during military operations".
The Taliban confirmed to the BBC that they had stolen the tankers, one of which became stuck at a river crossing.
"Villagers rushed to the fuel tanker with any available container that they had.
"There were 10 to 15 Taliban on top of the tanker. This was when they were bombed. Everyone around the fuel tanker died."
Mr Rasmussen said the leader of international troops in Afghanistan, Gen Stanley McChrystal, had told President Karzai he was committed to investigating the incident.
"It is really a focal point for our Isaf troops to minimise the number of civilian casualties, and a new strategy to that end has already been introduced," Mr Rasmussen said.
"Civilian casualties caused by Isaf are down over 95% from last year's levels. But, as we all know, in conflicts like these, mistakes can happen. In this case, let us now see what the investigation concludes."
The incident came days after Gen McChrystal reiterated calls for a fresh approach to the conflict.
"The situation in Afghanistan is serious, but success is achievable and demands a revised implementation strategy, commitment and resolve, and increased unity of effort," he wrote in a strategic assessment.
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