Raids by US aircraft in Kunduz province killed more than 30 civilians, including many women and children last week.
The United Nations said it was investigating an incident in which more than 30 civilians were killed in US air strikes called in support of a special forces raid on suspected Taliban fighters in northern Afghanistan.
The UN Assistance Mission in Afghanistan (UNAMA) said at least 32 people had been killed and 19 wounded in the strikes in the village of Buz Kandahari near Kunduz, the vast majority women and children.
The deaths add to a growing civilian casualty total in Afghanistan, where 95 have been killed and 111 injured in the past week alone, according to UN figures.
"The loss of civilian life is unacceptable and undermines efforts toward building peace and stability in Afghanistan," said Tadamichi Yamamoto, the secretary-general’s special representative for Afghanistan and head of UNAMA.
"When conducting aerial operations, international military forces should take all feasible measures to minimise civilian harm, including full analysis of the context for aerial strikes," he said in a statement.
The US military acknowledged on Saturday the air strikes had probably caused civilian casualties and promised an investigation. The top US commander in Afghanistan, General John Nicholson, expressed deep regret for the loss of innocent life.
The strikes were called in to protect a team of Afghan special forces and their US advisers who came under heavy fire during a raid on suspected Taliban commanders.
Three Afghan soldiers and two Americans were killed in the fighting.
Although US combat operations against the Taliban largely ended in 2014, special forces units have been repeatedly engaged in fighting while providing assistance to Afghan troops.
Thousands of US soldiers remain in Afghanistan as part of the NATO-led Resolute Support training and assistance mission and a separate counter-terrorism mission.
Afghan forces have suffered thousands of casualties, with more than 5,500 killed in the first eight months of 2016.
Last October, a US air strike hit a Doctors Without Borders (MSF) hospital in Kunduz, killing at least 42 people, including 24 patients, 14 staff and four caretakers.
At least 37 others were wounded in the raid, which destroyed the MSF hospital building and prompted widespread condemnation from human rights groups.
The US military said the air strike was a "mistake" and apologised, but insisted the attack was not a war crime.
- The air strikes were in support of special forces on the ground under Taliban fire, officials say