Pakistani security officials say they have arrested a teenager allegedly linked to last month's assassination of former Prime Minister Benazir Bhutto.

Intelligence officials, who asked not to be identified, say a 15-year-old boy, Aitezez Shah, was arrested in the northwestern city of Dera Ismail Khan late Thursday. They say he was preparing for a suicide attack at a Shi'ite mosque.

The officials say the boy confessed that he was part of a five-man team assigned to kill Ms. Bhutto, who was assassinated shortly after holding an election rally in Rawalpindi December 27.

A spokesman for Taliban and al-Qaida-linked militants, Mohammed Omar, denied any connection to the teenage boy and reiterated that the group was not involved in the attack on Ms. Bhutto.

Separately, Pakistani military officials say troops Saturday captured 50 militants in the tribal region of South Waziristan.

The arrests follow Friday's clashes that left up to 90 militants dead in the area along the Afghan border. The region is a stronghold for al-Qaida-linked militant leader Baitullah Mehsud, who both the Pakistani and U.S. governments say was behind the assassination of Ms. Bhutto.

Also Saturday, Pakistani police say they have arrested five men who were planning an attack against Shi'ites in the southern city of Karachi, using cyanide and suicide bombs.

Police say the men belonged to Sunni extremist groups. Security has been tightened across Pakistan due to threats of sectarian violence, as Shiites mourn the death of the Prophet Mohammed's grandson during the Muslim month of Muharram.

On Thursday, a suspected Sunni militant carried out a suicide attack at a Shi'ite mosque in Peshawar, killing at least 11 people.

Some information for this report was provided by AFP, AP and Reuters.