KABUL, Nov 20 (Reuters) - Afghan and U.S.-led coalition troops, backed by air power, killed a large number of Taliban fighters on Tuesday after an insurgent ambush in the south of the country, the U.S. military said.

The number of clashes and bomb attacks has risen in Afghanistan by up to 30 percent this year, according to United Nations estimates.

Despite suffering heavy casualties, the hardline Islamist Taliban has shown no let-up in its campaign to oust the pro-Western Afghan government and eject the 50,000 foreign troops from the country.

The latest clash came as a "company-size" group of Taliban insurgents ambushed a joint Afghan-coalition patrol with small arms fire and rocket-propelled grenades near the village of Sarsina, in the south-central province of Uruzgan.

In the U.S. army a company is usually around 100 troops.

The insurgents attempted to outflank the troops and moved into a nearby building and trench line, forcing civilians to flee. Troops then pinned down the Taliban fighters and called in air support.

"Several precision air strikes effectively eliminated the Taliban insurgents who were trying to reinforce the enemy positions," the U.S. military statement said. A "large number" of insurgents were killed, it said, without giving further details.

Winter has arrived late in Afghanistan this year and the traditional lull in fighting during the harsh weather has yet to take effect. Both sides in the conflict have vowed to keep up campaigning during the winter. (Writing by Jon Hemming, editing by Roger Crabb)

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