MARJAH, Afghanistan (Reuters) - A NATO airstrike in Afghanistan mistakenly killed 27 civilians, the government said on Monday, hurting a campaign to win over the local population and defeat Taliban insurgents.

A U.S. Marine from Bravo Company of the 1st Battalion, 6th Marines aims his weapon at Taliban fighters in Marjah in Helmand province, southern Afghanistan February 22, 2010. REUTERS/Goran Tomasevic

The Afghan cabinet condemned the deaths as “unjustifiable” after an aircraft fired on civilians, mistaking them for insurgents, in the south near the border of Uruzgan and Dai Kondi provinces.

But U.S. Defense Secretary Robert Gates defended work by the U.S. and NATO commander in Afghanistan to avoid killing civilians. He added that incidents like the airstrike were inherent in conflict and part of “what makes war so ugly.”

Civilian casualties have caused friction between the government and foreign forces, who have launched two big offensives in the past eight months to try to turn the tide of a growing Taliban-led insurgency.

Initially, the Afghan cabinet reported 33 deaths but later clarified that 27 people were killed. Sunday’s toll was still the highest number of civilian deaths in months.

‘TRUST AND CONFIDENCE’

The incident was not part of Operation Mushtarak, a major NATO-led campaign to clear Taliban militants out of neighboring Helmand province in the south.

But it could still undermine government and NATO efforts to win over civilians under a plan to wrest control of Taliban bastions and hand them over to state authorities before the start of a gradual U.S. troop withdrawal in 2011.

“Initial reports indicate that NATO fired Sunday on a convoy of three vehicles ... killing at least 27 civilians, including four women and one child, and injuring 12 others,” the Afghan cabinet said in a statement.

The NATO-led International Security Assistance Force (ISAF) said in a statement that civilians had been killed as they approached a joint NATO-Afghan unit but did not say how many.

An investigation has begun, it said.

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“We are extremely saddened by the tragic loss of innocent lives,” U.S. General Stanley McChrystal, commander of NATO forces in Afghanistan, said in the ISAF statement.

“I have made it clear to our forces that we are here to protect the Afghan people and inadvertently killing or injuring civilians undermines their trust and confidence in our mission.”

McChrystal’s counterinsurgency strategy emphasizes seizing population centers and avoiding combat in built-up areas whenever possible to avert civilian deaths. The number of civilians killed by NATO troops has fallen since he took command in mid-2009.

Brad Adams, Asia director for Human Rights Watch, praised McChrystal’s efforts to reduce civilian casualties and said they have made a difference but he said the coalition continues to act on poor intelligence.

“This has gone on for years. They urgently need to solve this problem to avoid mistakes that could undermine this policy,” Adams said.

SLOWER THAN EXPECTED

U.S. Marines say they have been extra careful not to put civilians at risk in their assault at Marjah in Helmand, Afghanistan’s most violent province. This has at times prevented them from acting more decisively.

U.S. officials acknowledged on Monday the Marjah operation was moving a bit slower than expected but Gates said he did not expect that to affect the timing of future operations to retake Taliban strongholds elsewhere.

Lieutenant General John Paxton, director of operations at the U.S. military’s Joint Staff, described the approach in clearing operations in Marjah as “very deliberate” to avoid killing civilians.

“We know this is going to be a hard fight,” Paxton said at a Senate hearing. “We know there are going to be pockets of intense resistance.”

The International Committee of the Red Cross said roads infested with hidden bombs make it difficult to evacuate the sick and wounded to hospitals in the provincial capital Lashkar Gah.

“The ICRC calls on the armed opposition, the Afghan National Army and police and international military forces to ensure that the conduct of military operations does not unduly affect access to medical care,” it said in a statement.

At least 12 people were killed in a poorly targeted rocket strike the day after Operation Mushtarak started this month. A total of 21 civilians have died in that NATO offensive, ISAF said.

While NATO forces appear to have made significant progress in the offensive -- a test of U.S. President Barack Obama’s troop surge strategy -- their push to clear out militants can backfire.

“People still complain about how the house searches are being conducted. The joint forces should not view every person here with suspicion of being a Taliban or a relative of one,” said Abdur Rahman Saber, head of a local council set up before the Marjah offensive to monitor the plight of civilians.

“When the government and its foreign allies want the people on their side, they should respect every resident here. People should not feel any sense of insecurity from Afghan or foreign troops.”

NATO and Afghan forces still are still under pressure to push out remaining Taliban fighters and prevent others from coming back to Marjah, a poppy cultivation center which Western countries say funds the insurgency.