"We deeply regret what occurred on yesterday's operation."
US army Col. Rafael Torres
Nato said it was sending a team to the area to determine what happened.
"We deeply regret what occurred on yesterday's operation," Col. Rafael Torres, a spokesman for Nato, said.
In July, a botched Nato airstrike killed six Afghan soldiers in Ghazni province in the east.
The coalition said the Afghan army unit gave the wrong location to international forces.
The helicopter crew received permission to launch the air strike because the headquarters believed the Afghan soldiers were in another area of the province at the time, the coalition said.
More civilians killed
Also in the north, five civilians were killed and two others were wounded when a bomb exploded on Saturday, the coalition reported.
In the west of the country, a woman and two children were accidentally killed on Friday in fighting that erupted as Nato and Afghan troops pursued a Taliban fighter who allegedly smuggled fighters and weapons from Iran.
The United Nations has reported the number of Afghan civilians killed or injured in the war soared to 31 per cent in the first six months of the year.
The UN said at least 1,271 Afghans were killed and 1,997 injured, mostly from bombings, in the first six months of the year.
General David Petraeus, the new US commander of Nato and US forces in Afghanistan, has stressed that reducing such casualties and protecting Afghans were his priorities.
The coalition also reported that two US soldiers died on Saturday and another was killed on Friday in attacks in the south. Nato did not disclose details of their deaths.
A fourth member of the coalition, a British serviceman, was killed on Saturday in Nad Ali district of Helmand, the British ministry of defence said.
Thirty-eight members of the international coalition, including 24 Americans, have been killed so far this month, according to a count by news agency The Associated Press.
In neighbouring Kandahar province, another police station was attacked on Friday in Kandahar city, according to the governor's office.
Two Taliban fighters were killed, but there were no police casualties.
Another six fighters were killed by Nato airstrikes as they got out of a vehicle and several other suspected Taliban fighters were detained during the operation in the Pusht Rod district of Farah province, the coalition said.