NATO mistakenly killed five of its Afghan army allies in an air strike Wednesday while the Afghans were attacking insurgents in the country’s east, officials said.

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 security to them nearly nine years into the war. Three American soldiers were also reported killed Wednesday in a roadside bomb in the south.

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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. Special Report: Afghanistan

Five Afghan soldiers died and two more were wounded in the air strike 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 air strike. 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.

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.

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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.

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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.

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 was expected to announce Wednesday that it would withdraw its troops from one tumultuous 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 a large portion of the 312 soldiers killed since 2001.

Britain’s defense ministry said Defense Secretary Liam Fox is to make a statement Wednesday to the House of Commons on the deployment of U.K. troops. A government official, who demanded anonymity to discuss the statement in advance, said U.S. troops are expected to replace U.K. forces in Sangin starting around November.

Britain has about 10,000 troops in Afghanistan, most based in Helmand.

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