Barack Obama has expressed "regret" for civilian deaths in Afghanistan, vowing to "make every effort" to avoid them as US-led forces step up their fight against the Taliban and al-Qaeda.
The US president's comments on Wednesday came in the wake of reports that a US-led air raid in Afghanistan this week had killed as many as 100 civilians.
If confirmed it would be the biggest loss of civilian life in a single attack since US troops invaded Afghanistan in 2001.
Rohul Amin, the governor of Farah province where the attack took place, said on Wednesday that he feared 100 civilians had been killed in the province's Bala Baluk district, about 600km from Kabul, the capital.
Earlier, Hillary Clinton, the US secretary of state, said in Washington that the Obama administration "deeply, deeply" regrets the loss of innocent life, calling such incidents as the reported killing of the 100 people "particularly painful".
Neither Obama nor Clinton specifically accepted US blame for the deaths.
Karzai, in the US capital for a US-Afghanistan-Pakistan meeting, thanked Clinton for "showing concern and regret" and said he hoped the two sides would work together completely to reduce civilian casualties in the "struggle against terrorism".
'Dozens of bodies'
An International Committee of the Red Cross (ICRC) team in Farah province's Bala Baluk district said it saw "dozens of bodies" at two locations.
"There were bodies, there were graves, and there were people burying bodies when we were there," said Jessica Barry, an ICRC spokesperson.
"We do confirm women and children."
Barry added that an ICRC community-based first aid volunteer and 13 members of his extended family including his five daughters and three sons, were among the dead.
She said that they were killed while sheltering in their home.
The US government has come under increasing criticism during the past year for civilian deaths during attacks against Taliban and al-Qaeda fighters in Afghanistan.
Barry, the ICRC spokesperson, said: "I think that it is important to remember that this is not a one-off situation. There has been a rise in casualties over the last year.
"It is absolutely important to remind all sides that civilians must not be harmed."
Karzai had earlier said that the civilian deaths in Farah were unacceptable and that he intended to discuss it with Obama.
The trilateral talks on Wednesday involving Karzai, Obama and Asif Ali Zardari, the Pakistan president, were aimed at addressing the war against the Taliban.
PHOTO CAPTION
US President Barack Obama (centre) sits alongside Afghanistan President Hamid Karzai (left) and Pakistani President Asif Ali Zardari during a meeting at the White House in Washington, DC on May 6.
Agencies