Eighteen Shiites were killed in cold blood in Pakistan's Kohistan region on Tuesday, in a sign of worsening sectarian strife in the Islamic republic.

Gunmen belonging to the Jundallah, an al-Qaeda and Taliban-linked terror group, had stopped a bus carrying the victims in the relatively quiet northern region of Pakistan. The gunmen, disguised as military personnel, hauled the Shiites from the bus and shot them dead near the town of Harban, which is inhabited by two Sunni tribes.

“The motive was sectarian. The gunmen were wearing army uniform,” Kohistan police chief Muhammad Ilyas said, according to the Daily Times.

The Jundallah claimed responsibility for the attack. They were Shias and our mujahideen shot them dead, a Jundallah commander said, according to Daily Times.

According to the Long War Journal, Jundallah has a history of targeting Shiites in the country. The two most high-profile attacks against Shia took place in May 2004, when Jundallah killed 38 Shia worshipers in separate attacks at the Hyderi Mosque and the Jinnah Road Mosque in Karachi, the report says.

Although the Jundallah terror outfit is based in Karachi, the Taliban-controlled tribal region of South Waziristan is one of its strongholds, and it maintains close ties with al-Qaeda.

Less than two weeks ago, a suicide bomber killed at least 26 people outside a mosque in a Shia neighborhood in Pakistan’s northwestern Kurram tribal region.

Kurram, the only part of Pakistan’s border region that has a significant Shia population, has been a theater of Sunni-Shia sectarian violence, the Dawn had reported.

“We have targeted the Shia community of Parachinar because they were involved in activities against us, Fazal Saeed, leader of a breakaway faction of the Pakistani Taliban, told Reuters.

Shia-Sunni conflict is widespread in Islam and goes back 1,400 years in history. The conflict is rooted in a dispute over the successor to Prophet Muhammad.

According to the CIA World Factbook, Sunni Islam accounts for over 75 percent of the world's Muslim population while Shia Islam represents 10-20 percent of Muslims worldwide.

The Shia believe Muhammad's cousin and son-in-law, Ali, was the only divinely ordained Imam (religious leader), while the Sunni maintain the first three caliphs after Muhammad were also legitimate authorities, says the Factbook.

In Pakistan, Sunnis constitute 75 percent of the population while Shiites account for 20 percent.

The sectarian underpinning of the Pakistani society has been clear from the time of the country's origin in 1947, but the conflict worsened during the reign of military strongman General Zia-ul-Haque in the eighties -- a time when the radical islamization of the society took place.

In recent times, the post-9/11 scenario worsened the strife. Some of the major attacks were the 2003 killing of 53 worshippers at the main Shia Friday Mosque in Quetta, the murder of 42 in an attack on a procession of the Shia Muslims in March 2004, the killing of 40 Shias in a suicide bombing in Karachi in December 2009 and the killing of at least 35 Shias in Lahore in September 2010.

In October 2004, 40 members of an extremist Sunni organization in Multan were killed in a car bomb.