By Mohammad Hashim

KOHAT, Pakistan, Jan 26 (Reuters) - A Pakistani army helicopter fired at militants trying to damage a road tunnel in the country's northwest on Saturday, killing up to 25, a senior government official said.

The helicopter fired at the militants as they gathered to damage the Kohat Tunnel, which links the towns of Kohat and Darra Adam Kheil in the Frontier Province, the official said.

"Some 20-25 militants have been killed in the attack," said Kamran Zeb, District Coordination Officer of the Kohat region.

A military spokesman confirmed the clashes in the area.

The incident came a day after up to 30 pro-Taliban militants and two soldiers were killed in Darra Adam Kheil in fighting that erupted when insurgents seized four trucks carrying ammunition and other supplies for troops on Thursday.

Known as a haven for smugglers, the small dusty town of Darra Adam Kheil had until recently been relatively free of the militant violence that has plagued other parts of Pakistan's tribal belt along the border with Afghanistan.

Security forces also battled militants in South Waziristan, known as a sanctuary for al Qaeda militants.

Nearly 150 militants and more than 20 soldiers have been killed in South Waziristan in a week of clashes with fighters loyal to Baitullah Mehsud, a Taliban commander the government says is linked to the assassination of opposition leader Benazir Bhutto, last month.

Mehsud, who has also been blamed for a string of suicide bomb attacks, ordered his fellow Taliban commanders on Friday to step up attacks on Pakistani security forces, according to a spokesman for the militant commander.

(Writing by Augustine Anthony; editing by Diana Abdallah)

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