Aghanistan government officials say at least 33 civilians have been killed in a Nato attack on a convoy of vehicles in Uruzgan, a province in the country's south.
Nato confirmed that it fired on Sunday on a group of vehicles that it believed contained fighters, only to discover later that women and children were in the cars.
Isaf, Nato's force in Afghanistan, did not provide a figure of how many died.
Earlier, Amanullah Hothaki, the head of the provincial council for Uruzgan, said 19 people were killed in the attack, which hit three vehicles as they drove down a main road.
A 15,000-strong joint force of Afghan, Nato and US troops is battling the Taliban in Marjah, a town in neighbouring Helmand province, where the fighters have been in control for years.
'Clear mistake'
Al Jazeera's James Bays, reporting from Lashkar Gah in Helmand on Monday, said an "aerial weapons team" fired at vehicles carrying civilians.
"We understand they [civilians] came under attack from what Nato says was an aerial weapons team," he said.
"That sounds to me like helicopters. The Nato spokesman I have spoken to ... says there was a Nato operation against the Taliban in the nearby area, and Nato thought they were reinforcements for the Taliban coming.
"And that's why they launched this air strike. But he says it's clear now it was clearly a mistake, and that in many of these vehicles there were women and children."
General Stanley McChrystal, commander of the international forces in Afghanistan, released a statement apologising for the deaths, our correspondent said.
"We are extremely saddened by the tragic loss of innocent lives," McChrystal was quoted by Bays as saying.
"I've made it clear to our forces that we're here to protect the Afghan people and inadvertentlly killing and injuring civilians undermines their trust and confidence in our mission. We'll redouble our efforts to regain that trust."
The deaths in Uruzgan come close on the heels of an emotional appeal by Hamid Karzai, Afghanistan's president, for international troops to try harder to prevent civilian deaths.
He told parliament on Saturday that although progress was being made in limiting civilian casualties, people were still dying.
He held up a picture of an eight-year-old girl who lost 12 relatives in a Nato rocket attack during the second day of the assault on Marjah - Operation Moshtarak - which began on February 13.