KABUL, Dec 12 (Reuters) - A NATO-led foot patrol killed three Afghan civilians and wounded 11 others on Friday after opening fire on two vehicles in Wardak province, southwest of Kabul, the alliance said.

The foot patrol opened fire on a car and minibus after they failed to stop when the foreign soldiers fired warning shots on the highway between Kabul and the southern province of Kandahar, a spokesman for NATO in Kabul said.

Immediately after the incident the foot patrol was ambushed while troops were treating the casualties, the NATO spokesman said.

Afghanistan has suffered its worst violence this year since U.S.-led and Afghan forces overthrew the Taliban in 2001, with at least 4,000 people killed, around a third of them civilians.

The casualties would add to tension between the Afghan government and its Western backers, already high after the deaths of dozens of civilians in a string of mistaken air strikes this year. (Reporting by Kabul newsroom; Editing by Jerry Norton)

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