At least 30 people were killed in sectarian violence in Pakistan's volatile tribal Kurram Agency, officials said.
This brings the death toll to 43 in clashes that broke out over the weekend. "Five people, including three children and a woman, from a Shia group died in Sadda village when a mortar round hit a school building where they had taken refuge," local official Mujahid Khan said on Wednesday.
The mortar shell had been fired by the Sunni tribesmen reportedly supported by well-trained pro-Taliban militants. The assailants also targeted a security check post in the same area with rockets and killed two paramilitary troops, Khan added.
Ten more people were killed in various exchanges of fire, while 13 pro-Taliban fighters who had come to the area from neighbouring tribal districts to aid Sunni tribesmen were also killed while fighting the security forces.
The clashes in the region triggered last Saturday when armed Sunni tribesmen entered Parachinar city, the main urban settlement in the Kurram district, and ransacked shops and houses of Shia Muslims. At least 13 people were killed in the ensuing fighting.
Kurram district has a long history of sectarian violence. Weeklong fierce clashes left more than 150 people, including 13 security personnel dead in November. Around 400 more were injured.
Shias account for 20 per cent of Pakistan's population of 160 million, but are in the majority in Parachinar.
In a separate development, Pakistan's army claimed it had arrested 18 militants in the neighbouring North West Frontier Province's volatile Swat valley, where pro-Taliban cleric Maulana Fazlullah has incited an armed campaign to enforce Taliban-style Islamic rule in the area.
"These people were involved in serious terrorist activities," said the army's chief spokesman Major General Waheed Arshad.
The military said early this month that it had cleared most parts of the valley under the operation that started in late October, killing 290 and arresting more than 140 in a seven-week offensive. More than a dozen soldiers also died in the action.
But Fazlullah is still at large and his followers are carrying out guerrilla raids on the security forces. At least 10 people, including four soldiers, were killed Sunday when an attacker rammed his explosives-laden vehicle in a military convoy in Mangora, the main town in the scenic valley.