MIRANSHAH (AFP) - Troops backed by attack helicopters clashed with Taliban fighters in the main town of North Waziristan tribal district on Wednesday, witnesses and officials said. The rare clashes came one day after a bomb killed three soldiers and although military officials confirmed troops were in action, there was no sign it was the start of a major operation, long demanded by Washington. North Waziristan, the most infamous of seven tribal districts on the Afghan border, is a stronghold of the Al-Qaeda-linked Haqqani network, but Pakistan has resisted US pressure to launch a sweeping offensive in the area. Instead witnesses said Wednesdays clashes broke out after troops started to blow up a private hospital used by the Taliban and other militants, one day after a nearby bomb attack killed three troops and wounded another 15. One local resident said he saw two gunship helicopters shelling a government school, where militants were holed up, targeting soldiers. Another witness said militants fired on a helicopter from a roof top in the main market, where traders were trapped by the fighting and locals said they feared civilian casualties. The gunship shelled three times in their direction, a shopkeeper trapped by the clashes told AFP by telephone. An AFP reporter heard several blasts and saw several Taliban fighters firing on army checkposts with automatic weapons and rocket launchers in the town of Miranshah. The market shut and the town was plunged into a blackout after Taliban militants targeted an electricity transformer, the reporter said. It was not a planned operation. We moved ground troops to blow the hospital up and in the meantime, militants attacked the soldiers, a security official told AFP in Peshawar. The decision to dynamite the hospital was taken after yesterdays bomb attack. The exchange of fire is continuing. Gunships (helicopters) are also shelling them, he added. Authorities ordered an indefinite curfew and told residents to stay at home, in announcements made throughout the afternoon by loudspeaker on mosques as the clashes, at times heavy, continued into a fifth hour. In one street, a witness said he saw six Taliban fire on troops and saw one fighter being shot and killed. The same witness said the militants were talking Uzbek and Urdu, indicating that they came from Uzbekistan and Punjab. Pakistan has been under huge pressure to do more to destroy militant sanctuaries, particularly in North Waziristan. But Pakistan has said any North Waziristan offensive would be of its choosing, arguing that its 140,000 troops already committed to the northwest are too overstretched fighting a homegrown insurgency to take on the Haqqanis.