KABUL: A NATO air strike killed up to 33 Afghan civilians, including women and a child, sparking fresh anger from Kabul on Monday against US-led forces pressing a major offensive to defeat the Taliban.

Top US commander Stanley McChrystal, who has made winning Afghan hearts and minds the focus of plans to end the eight-year war in Afghanistan, was forced into another apology over civilian deaths after the third incident in a week.

"We are extremely saddened by the tragic loss of innocent lives. 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. We will re-double our efforts to regain that trust," McChrystal said.

McChrystal spoke to Karzai on Sunday, expressing his "sorrow and regret for the tragic incident" and pledging to fully partner with the joint investigation, NATO said in a statement.

McChrystal and his superior, General David Petraeus, mapped out an offensive lasting 12-18 months that would strike beyond the current focus of operations in the southern province of Helmand.

But the third mistaken NATO air strike reported by Afghan officials in a week risked undermining the campaign's strategic goals.

The government said four women and a child were among the civilians killed in Gujran district, Daykundi province, on Sunday when NATO forces mistook their convoy for Taliban militants.

The air raid, which also injured 12 people, came days after a NATO rocket attack on a house killed at least nine Afghan civilians -- for which General McChrystal also apologised.

Afghanistan's council of ministers, chaired by President Hamid Karzai, condemned the latest incident as "unjustifiable".

In Kapisa province, just north of Kabul, militants fired a rocket-propelled grenade into a sedan vehicle Monday, killing another civilian during a clash with military forces, the interior ministry said.

Civilian casualties are a sensitive issue in Afghanistan, where President Hamid Karzai and his Western backers are trying to win a war of perceptions.

Last Thursday, a NATO bombing raid in the northern province of Kunduz killed seven Afghan policemen, according to hospital and government officials.

On February 15, NATO acknowledged that five civilians were killed accidentally and two others wounded in an air strike in southern Afghanistan.

Karzai used Saturday's opening session of parliament to repeat his call for civilians to be protected as 15,000 Afghan, US and NATO troops press Operation Mushtarak (Together) in the southern province of Helmand into a second week.

The assault on the Marjah and Nad Ali areas in the heartland of southern Afghanistan's poppy growing region is the first step of a wider campaign that will last 12-18 months, McChrystal and Petraeus say.

The operation is the first key test of US President Barack Obama's strategy to end the war by boosting the number of US and NATO troops in Afghanistan to around 150,000 by August and stepping up development.

The military phase of the offensive, now into a ninth day, are due to be followed by efforts to reassert government control with security and services.

Afghan police have moved into the target area, but commanders say it could be another month before it is cleared of fighters and their booby trap bombs.

NATO on Monday described resistance from Taliban fighters as "determined" in Marjah while "cautious optimism" was the order of the day in nearby Nad Ali, "as early signs indicate a return to normality".

McChrystal was quoted as saying that Kandahar province, neighbouring Helmand and the spiritual home of the Taliban, is the likely next target of operations to eradicate the insurgent militia.

Mushtarak was a "model for the future," he was quoted by Britain's The Times newspaper as telling reporters in Kabul in the first comments to acknowledge a wider theatre for NATO operations.

"We are going to go to where significant parts of the population are at risk and Kandahar is clearly very, very important not just to the south but to the nation," he said, adding: "It is not the only area though."

"This is just the initial operation of what will be a 12-18 month campaign as General McChrystal and his team mapped it out," Petraeus told US television on Sunday, describing the Taliban resistance as formidable but disjointed.

NATO said Monday a bomb attack killed two soldiers in southern Afghanistan, bringing to at least 93 the number of foreign soldiers to have died in the country so far this year, according to an AFP count.