By Mohammad Hashim

KOHAT, Pakistan, July 17 (Reuters) - Pakistani security forces backed by gunship helicopters cleared two northwestern towns of Taliban militants after 15 soldiers were killed in an ambush last weekend, the military said on Thursday.

The action launched late on Wednesday cleared militant nests and secured an irrigation dam and fort north of Hangu, 40 km (25 miles) west of the garrison town of Kohat.

"There's normal resistance and now our troops are advancing," spokesman Major-General Athar Abbas said, adding he had no information on casualties.

Residents said intermittent mortar fire could be heard, but intelligence and government officials, speaking on condition of anonymity, said most militants had already fled to Orakzai, a neighbouring tribal region.

A general deterioration in the security situation across the northwest in recent weeks has coincided with calls by Western allies, with troops in neighbouring Afghanistan, for Pakistan to put the militants under greater military pressure, while also engaging the less recalcitrant groups in dialogue.

Authorities imposed a curfew in and around Hangu and distributed leaflets asking residents in some parts of the town and its surroundings to move to relief camps.

"We're scared what is going to happen. They should immediately lift curfew. We can't leave our houses like this," said a resident Malik Asghar.

Hangu has long been plagued by militancy, but tension heightened last week after militants kidnapped paramilitary soldiers and government workers.

They have threatened to kill 49 hostages if seven comrades were not freed.

"It depends on the government. If they free our people and stop the operation, we'll release hostages and stop our activities," Taliban spokesman Maulvi Omar told Reuters by satellite telephone from an undisclosed location.

Violence had subsided in Pakistan's northwest after a new coalition, formed following February elections, had pursued a policy of dialogue with the militants.

But the Pakistani Taliban leader Baitullah Mehsud suspended talks last month and the security situation has deteriorated once more.

Authorities named Mehsud in a wave of suicide attacks across the country over the past one year, including the one that killed two time prime minister Benazir Bhutto, whose party heads the coalition. Mehsud has denied involvement. (Additional reporting and writing by Kamran Haider; editing by Simon Cameron-Moore and Jerry Norton)

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