US air strikes in Afghanistan on Tuesday killed dozens of civilians including women and children, officials from the Red Cross have said.
Afghan officials in the western province of Farah told the BBC as many as 100 civilians might have died.
The civilians were said to have been hit while sheltering from fighting.
A US military team has been sent to Farah to investigate and Afghan President Hamid Karzai has ordered an Afghan inquiry.
Mr Karzai is in the US for talks with President Barack Obama and Pakistani President Asif Ali Zardari.
Civilian deaths will be high on the agenda at the White House for Mr Karzai, who has repeatedly urged Western forces in Afghanistan to reduce the number of civilian casualties.
The BBC's Martin Patience, in Kabul, says the Washington talks could be overshadowed if the Red Cross report of dozens of civilian deaths is confirmed.
Separately, US defence secretary Robert Gates arrived in Kabul on an unannounced visit. An AFP news agency correspondent travelling with him says he will focus on preparations for the scheduled influx of tens of thousands more US troops.
International Committee of the Red Cross observers who visited the site of the air strikes saw houses destroyed and dozens of dead bodies, including women and children, a spokeswoman said.
"We can absolutely confirm there were civilian casualties," Jessica Barry said.
"It seemed they were trying to shelter in houses when they were hit."
The governor of Farah province, Rohul Amin, backed the Red Cross verdict that civilians had died but he could not confirm numbers.
The US military said coalition troops had been called to assist Afghan forces as they attempted to fight off an insurgent attack.
A spokeswoman, Capt Elizabeth Mathias, said she was "extremely concerned" by the reports of high casualties.
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