NATO mistakenly killed five of its Afghan army allies in an airstrike Wednesday while they were attacking insurgents in the country's east, officials said.
Three American soldiers were also reported killed Wednesday in a roadside bombing in the south.
An Afghan defense official condemned the latest "friendly fire" deaths, which came at a time when international troops are trying to improve coordination with Afghan security forces in hopes of handing over more responsibility to them nearly nine years into the war.
The international alliance later apologized, saying one of its patrols in the area mistook the Afghan soldiers for insurgents and targeted them with precision-guided munitions. NATO said in a statement that a joint Afghan-international team was continuing to investigate.
The Afghan soldiers were launching an ambush before dawn against insurgents reportedly on the move in Ghazni province when NATO aircraft began firing on them without warning, Afghan Defense Ministry spokesman Gen. Mohammad Zahir Azimi said.
Five Afghan soldiers died and two more were wounded in the airstrike in Ghazni's Andar district, he said.
"This is not the first time such an incident has happened, but we wish that at least this would be the last one," Azimi said.
NATO spokesman Josef Blotz extended the personal condolences of U.S. Gen. David Petraeus, the newly arrived commander of NATO forces in Afghanistan, to the families of the victims.
"The reason for this is perhaps a coordination issue," Blotz said. "We were obviously not absolutely clear whether there were Afghan national security forces in the area."
The Afghan soldiers' deaths were another setback in the U.S.-led force's goal of training and coordinating with the Afghans, one of the cornerstones of its counterinsurgency strategy.
NATO is counting on the strategy to beat back the insurgents' recent gains, nearly nine years after U.S.-backed forces toppled the Taliban's hard-line Islamist regime. The aim is to win over the population by limiting Afghan casualties while securing new areas, eventually turning control over to local army and police and allowing foreign troops to withdraw.
The errant air strike comes as U.S. troops ramp-up a highly risky and strategically vital battle to reclaim the Taliban's hometown from the militant group.
The long-awaited Kandahar operation is finally under way, reports CBS News correspondent Mandy Clark. The first step is simply to get the city under control, and it has U.S. troops patrolling the streets more like police officers than warriors.
The easy part is rebuilding the police stations and teaching the cops how to do their job. The harder part is setting up checkpoints on every main road leading in and out of the city.
Each of the 13 roadblocks will be manned by a company of Afghan Police and an equal number of American military police who will live and work together, and incidents like the NATO air strike on Wednesday won't do anything to help the atmosphere.
Also on Wednesday, NATO said three American troops were killed by a roadside bomb in the south Tuesday. It did not identify them or give any other details.
Last month was the deadliest for international forces since the war began, with 103 killed, including 60 Americans.
Britain announced Wednesday that it will withdraw its troops from a volatile district in the south, turning over responsibility to U.S. forces. The Sangin valley in Helmand province has been the deadliest area for British forces, accounting for 99 of the 312 soldiers killed since 2001.
Britain's military said U.S. forces would move into Sangin from October. It has about 10,000 troops in Afghanistan, most based in Helmand.
The international forces' commanders on Wednesday portrayed the redeployment as a tactical move that would allow British forces to move into central Helmand to beef up security there.
"These additional forces almost 300 in total will give commanders additional flexibility over the summer to reinforce progress in central Helmand," said U.S. Army Lt. Gen. David M. Rodriguez.
Another NATO airstrike on Tuesday in eastern Paktika province killed several suspected insurgents and led to the arrest of several others.
NATO said a coalition aircraft fired a precision-guided munition to repel militants who were firing at Afghan and coalition forces from an area near an unmarked mosque in Yahya Khel district. Windows of the mosque were broken in the fighting.
When the clash ended, the patrol recovered six hand grenades, three AK-47 assault rifles, a sniper rifle, a rocket-propelled grenade and a book on how to make improvised explosive devices, NATO said. It said the combined force later discussed the incident with local elders and government officials.
Two times in the past five weeks, coalition and Afghan forces have been engaged in fights with insurgents in or near mosques. NATO said that on June 3 in Wardak province, a joint force received fire from two mosques where they found a weapons cache and communications equipment. On July 4 in Kunduz province, a joint force discovered rocket-propelled grenades, automatic weapons and a roadside bomb near a mosque.