A Red Cross spokeswoman said they were investigating the deaths
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.
US Secretary of State Hillary Clinton said at a meeting with the Afghan president that America "deeply, deeply regretted" the reported deaths.
Hamid Karzai, speaking alongside her and Pakistani President Asif Ali Zardari in Washington, thanked Mrs Clinton for her remarks. Earlier, the Afghan leader ordered an inquiry into the incident in Farah.
More than 40,000 have migrated from Mingora since Tuesday afternoon
Mr Zardari, who faces a Taleban insurgency of his own, said Pakistan would share the burden of defeating terrorism.
Meanwhile, US Defense 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.
BBC News, Kabul
Here in Afghanistan, people are taking this very seriously. Just a few hours ago there were rowdy scenes in the Afghan parliament with many parliamentarians protesting at what appears to be a large loss of lives of Afghan civilians.
Ultimately, the international community and the Afghan government need to win over ordinary Afghans. Every time there is an incident like this, it sets back the whole agenda.
So it's a very dangerous, problematic step for the international community and something the international forces will be desperate to avoid in the future.
"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.
"I actually sent an investigation team out to that region this morning and I expect them to be on the ground a little bit later this afternoon, and hopefully have some more information for us at that time," she said.
Tuesday's violence broke out after more than 100 Taleban militants attacked a police checkpoint in Farah, in the far west of Afghanistan, killing three police, Afghan officials say.
They later entered a village where they beheaded three people accused of being government collaborators.
US air strikes targeted militants thought to be sheltering in houses and at least 25 Taleban fighters are reported to have died.
But a growing number of reports from the area suggest civilians had also been sheltering in the buildings hit.
Local officials initially told BBC correspondent Martin Patience in Kabul they had seen the bodies of about 20 women and children in two trucks.
But officials and police sources in Farah later estimated the number of dead at around 100.
Opinion polls have suggested that civilian casualties are the single biggest concern cited by ordinary Afghans when asked about the continuing presence of international troops in the country.
Last year saw a 40% rise in the number of civilian deaths to more than 2,100. Although most were victims of the Taleban, more than 550 were the result of air strikes by US and Nato-led forces.
Crisis in Pakistan
Mr Obama will hold bilateral talks with Mr Karzai and Mr Zardari, before all three hold a joint meeting.
Mr Zardari faces a growing crisis in his own country amid a new outbreak of fighting between the army and Taleban rebels in the Swat Valley region.
Thousands of residents there are reported to be fleeing their homes as a peace deal between the government and Taleban militants appears on the verge of collapse.
Fighting flared overnight in Mingora, the main town in Swat, and continued into Wednesday, with reports of helicopter gunships bombarding militant positions.
The government has warned that 500,000 people could try to leave if the peace deal formally breaks down, although the BBC's Mark Dummett, in Islamabad, says the army has not yet launched the offensive most are now expecting.
On Tuesday the US envoy to Afghanistan and Pakistan, Richard Holbrooke, told a congressional hearing in Washington that Pakistan must do more to combat the Taleban.
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