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 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 confirmed the botched airstrike. He said he regretted the Afghan National Army deaths, telling a news briefing that a joint investigation has been launched.
"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."
He 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 Afghan soldiers' deaths at the hands of their allies was 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.
Violence has been increasing across Afghanistan, coinciding with the arrival of thousands of American soldiers for a new push to try to establish Afghan government control in the south, one of the Taliban's strongest areas of influence.
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 one of the deadliest for British forces, accounting for 99 of the 312 soldiers killed since 2001.
Defense Secretary Liam Fox made the announcement in a statement to the House of Commons on the deployment of British troops. Britain's military said U.S. forces would move into Sangin from October.
"In Sangin, U.K. forces have made huge progress in the face of great adversity," Fox told the lawmakers.
Britain has about 10,000 troops in Afghanistan, most based in Helmand.
Another NATO airstrike on Tuesday in 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.
Associated Press Writer David Stringer contributed to this report from London.