DERA ISMAIL KHAN, Pakistan - Pakistani security forces killed dozens of militants in South Waziristan on Friday, the army said, as violence escalated sharply along the Afghan border following Taliban assaults on two strategic military outposts earlier this week.

A large number of militants gathered to the north of another fort at Ladha after attacking it with small arms and rockets. Security forces responded with artillery, mortar and small arms fire, killing between 50 and 60 militants and forcing the rest to disperse, an army statement said.

Also Friday, a convoy of security forces came under attack near the village of Chakmalai, sparking a one-hour clash. The army estimated that between 20 and 30 militants were killed. Four soldierss were wounded.

It was not immediately possible to independently confirm the casualty figures.

Earlier Friday, a medic in Chakmalai reported heavy fighting after a large number of infantrymen moved into the area under the protection of helicopter gunships -- although the army spokesman denied it had launched an offensive. The medic, Alam Sher, said two people were killed close to the village.

"Since early morning I have been hearing gunshots and explosions, and I am receiving calls from local people to come to provide medical aid to the injured,'' he said, adding that nobody could get to the two bodies because the gunfire was continuous.

An intelligence official in the region confirmed that fighting was going on in Chakmalai and that the army and paramilitary Frontier Constabulary were trying to clear the area of pro-Taliban insurgents.

"Security troops moved in after fighting the militants, and they have taken up positions at strategic locations to prevent militants from operating from this area,'' said the official, who asked not to be identified because he was not authorized to speak to the media.

On Thursday, the army reported Cobra gunships fired on two vehicles carrying rebels, killing eight.

The fighting follows assaults by hundreds of troops loyal to Taliban commander Baitullah Mehsud on two military forts this week in South Waziristan that highlighted rising rebel control over the area by al Qaeda and Taliban fighters blamed for a surge in suicide attacks in both Pakistan and Afghanistan.

In other violence, a suspected Sunni extremist blew himself up inside a Shiite mosque, killing 11 people in the northwestern city of Peshawar. The strike late Thursday wounded 25 people, including a prominent Shiite cleric, ahead of this weekend's Ashoura festival, which often is scarred by sectarian violence.

The attack added to tensions in the country ahead of Feb. 18 parliamentary elections that many predict will weaken President Pervez Musharraf's grip on power eight years after he seized control of the nuclear-armed nation in a military coup.

In a related development, two Sunni extremists on death row for a 2004 attack on a procession that killed 42 Shiites were found missing from their cell Friday in Quetta in the southwest province of Baluchistan, said city police chief Rehmatullah Niazi.

There was no sign of a forced exit and several jail officials were being questioned over the escape, he said.

Militants have launched a wave of suicide bombings against security forces and politicians in recent months, killing at least 400 people, including the Dec. 27 gun and bomb attack that killed secular opposition leader Benazir Bhutto, who had vowed to battle Islamic extremism.