(Adds civilian casualties)

By Hamid Shalizi

KABUL, July 28 (Reuters) - Taliban insurgents killed three NATO troops and an Afghan soldier in two separate clashes in Afghanistan, the alliance and an Afghan official said on Saturday.

Two U.S. soldiers from the NATO force in Afghanistan and the Afghan were killed on Friday in an area of eastern Nuristan province in a clash with Taliban rebels. Thirteen NATO soldiers were wounded in the fighting.

The alliance said 24 insurgents were also killed in the clashes, close to the border with Pakistan. Fighting was continuing on Saturday morning, provincial governor Tameem Nuristani said, but he declined to give further details.

A Taliban spokesman said only three insurgents were killed in the clashes and said the casualties of NATO and Afghan troops were higher than reported.

During the fighting, a NATO airstrike hit a civilian vehicle in the Kamdish district of Nuristan province killing four people and wounding seven others, said a provincial official who declined to be named.

"Eleven passengers were travelling in a pick-up truck ... to Kamdish when it came under attack from the air," the official said. "Among the seven wounded civilians, two of them are in a serious condition."

President Hamid Karzai has warned international forces in Afghanistan that civilian casualties risk alienating support for the presence of foreign troops in his country.

A British soldier in the NATO force was killed in another clash in the south of the country on Friday. He was the third British soldier killed in three days in the south, one of the main strongholds for the resurgent Taliban.

Also on Friday, six Taliban rebels and an Afghan working for a U.S. security company were killed in a clash in the western province of Farah, the Interior Ministry said.

Violence has surged in Afghanistan in the past 18 months, the bloodiest period since the Taliban's overthrow in 2001.

Separately, four Afghan police were killed in an ambush on Saturday on a road in Logar province, which lies to the south of the capital Kabul, provincial police said.

Friday's deaths bring the number of foreign forces killed in action to more than 90 this year in Afghanistan. (Additional reporting by Mohammad Obaid in Logar)

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