Pakistani troops backed by attack helicopters clashed with Taliban fighters in the main town of the notorious North Waziristan tribal district on Wednesday, witnesses and officials said. The rare clashes came one day after a bomb killed three Pakistani 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 Pakistan's 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 Wednesday's clashes broke out after Pakistani 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 Pakistani 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 Pakistan army checkposts with automatic weapons and rocket launchers in the town of Miranshah, 300 kilometres (around 200 miles) southwest of Islamabad. 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 the main northwestern city of Peshawar. "The decision to dynamite the hospital was taken after yesterday's 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 Pakistani 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 Pakistan's central province of Punjab. Pakistan has been under huge pressure to do more to destroy militant sanctuaries, particularly in North Waziristan, since US Navy SEALs killed Osama bin Laden in the town of Abbottabad on May 2. The military this week announced the start of a new operation in the tribal district of Kurram, which has been plagued by sectarian clashes between Shiite and Sunni Muslims, which local officials say has displaced 28,000 people. 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. The army chief of staff, General Ashfaq Kayani, a close US ally who has been under huge strain since the bin Laden raid, on Wednesday called Pakistan's commitment to the war on Al-Qaeda and its affiliates "total and unwavering"."Pakistan firmly believes in taking stern action against all terrorist groups," he told a de-radicalisation conference in the northwestern valley of Swat, where the military expelled a Taliban insurgency two years ago. North Waziristan is home to around 400,000 people and Miranshah 50,000. US drone strikes, which are controversial among an anti-American Pakistani public, routinely target Taliban and Al-Qaeda-linked commanders in the district. Washington has called Pakistan's semi-autonomous northwest tribal region the most dangerous place on Earth and the global headquarters of Al-Qaeda. On Wednesday, Pakistani officials again accused several hundred militants of infiltrating the Afghan border and attacking a village in the district of Upper Dir, killing an anti-Taliban elder and setting fire to three boys schools. "The village militia and Pakistan troops are retaliating," district police chief Mir Qasim Khan told AFP.