A suicide bomber slammed an explosive-laden car into this passenger bus carrying Shiite pilgrims in Akhtarabad, on the outskirts of Quetta, on Jan. 1. (NASEER AHMED/REUTERS)

A surge in sectarian killings is raising new fears about Pakistan’s stability, as violence against Shiites and other minorities spreads to major cities and increasingly targets the country’s professional class.

Although Pakistan has struggled for decades with bouts of sectarian violence, the death toll rose dramatically last year and the country is experiencing a gruesome start to 2014.

Masked gunmen have been stalking Shiite doctors, lawyers and college professors. On Jan. 6, a suicide bomber tried to enter a school filled with several hundred students in a Shiite-dominated area in the northwest but was stopped by a ninth-grader, who is being hailed as a national hero after he died when the bomb went off in the ensuing scuffle. Other religious minorities, including Sufi Muslims, are facing lethal assaults.

The turmoil occurs at a critical moment. U.S. leaders are hoping that Pakistan can help maintain regional stability this year as most NATO troops withdraw from neighboring Afghanistan. But observers say any sectarian tension in Pakistan could easily spill over into Afghanistan, where security remains perilous and where religious and ethnic rivalries simmer, too.

“Nothing is going to get better, and it’s probably going to get worse,” said Sheikh Waqas Akram, a former member of Parliament from the eastern province of Punjab, where sectarian tensions have been on the rise.

There were 687 sectarian killings in the country last year, a 22 percent increase over 2012, according to the Pakistan Institute for Peace Studies. Although the deaths represented just a small portion of the toll of violence and terrorism in Pakistan — which claimed 4,725 lives last year — sectarian unrest is spreading throughout the country and becoming routine in heavily populated areas, the group concluded.

About three-fourths of Pakistanis are Sunni Muslims, while Shiites make up 15 to 20 percent of the population. Despite attacks over the years by Sunni militants, Pakistan has largely avoided the sectarian strife that has plunged Iraq and Syria into turmoil.

But analysts and some Pakistani political leaders are increasingly questioning whether Prime Minister Nawaz Sharif can keep order in the nuclear-armed country of 180 million people.

“We are on a very dangerous trend where sectarian violence is increasing, and it is starting to take the shape of structural violence,” said Muhammad Amir Rana, director of the Pakistan Institute for Peace Studies. “We are now seeing sectarian tensions triggered not only by terrorism incidents, but average clashes within the sectarian communities.”

Pakistan’s largest city, Karachi, was rattled this month when six men were found executed near a Sufi shrine. All of the victims’ throats had been slashed, and at least two of the men had been beheaded. A note was found next to their bodies warning others not to visit the shrine. The Pakistani Taliban took credit for the attack.

A few days later, in the northwestern city of Mardan, two men were fatally shot as they slept in a Sufi shrine. Sufi Muslims practice a mystical form of Islam and have been targeted for years by Islamist extremists.

Meanwhile, Shiite professionals have increasingly been targets of assassination attempts. Among the victims last year were a prominent poet in Karachi, a well-
respected doctor in the eastern city of Lahore and a university leader in the eastern city of Gujrat. Extremists are apparently trying to intimidate educated Shiites into leaving the country — a “brain drain by force,” said Salman Zaidi, deputy director of the Jinnah Institute, an Islamabad-based think tank.

The attacks on Shiites have continued in the new year. On Jan. 5, a 59-year-old Shiite doctor was fatally shot as he traveled home from his hospital in Multan in Punjab province. Two days later, a Shiite bank branch manager was fatally shot in the northwestern city of Peshawar, according to Pakistani news media reports.

“The attacks are getting more and more brazen,” Zaidi said. “There is a very real sense that the state will not be able to protect the Shia community, and it’s not just the Shias.”

Pervaiz Rashid, Pakistan’s information minister, disputed such conclusions, saying the government is cracking down on “sectarian outfits” and recently launched raids against militants in Karachi.

“No one will be allowed to destabilize Pakistan,” Rashid said in an interview.

For much of Pakistan’s 66-year-old history, tension between Shiite and Sunni communities was rare; the country’s founder, Muhammad Ali Jinnah, stressed tolerance. But the government permitted the formation of Sunni militant groups in the 1980s as backstops against Shiite-
dominated Iran and majority-Hindu India.

Pakistani officials said sectarian violence intensified in the aftermath of the Sept. 11, 2001, terrorist attacks on the United States, when the Taliban regime was ousted in Afghanistan and its fighters crossed into Pakistan.

As the border became less stable, hundreds of thousands of people sought refuge in Karachi, Lahore and other Pakistani cities. The influx has meant that hard-line Shiites and Sunnis compete for space in heavily populated areas.

“There was no issue with Sunnis and Shiites in our district before 2007,” said Gulab Hussain Tori, a Shiite leader in Peshawar. “We were like brothers, but, unfortunately, the situation changed since 9/11 and the arrival of militants.”

According to the South Asia Terrorism Portal, which monitors violence in the region, Pakistan’s death toll from sectarian violence last year was the highest since the organization began tracking the statistic in 1989, when 18 people were killed. That number has grown to more than 500 for each of the past two years.

Pakistani leaders and observers say an especially troubling development occurred in mid-November, when Sunnis and Shiites clashed in Rawalpindi, a garrison city adjacent to Islamabad.

The fighting broke out as Shiites participating in a religious holiday procession marched past a mosque where a hard-line Sunni cleric was delivering a Friday sermon. A cloth market and nearly 100 shops were set on fire. Police said 10 people were killed, although residents said at least twice that number died.

Knox Thames, director of policy and research at the U.S. Commission on International Religious Freedom, said Western officials and humanitarian groups are still awaiting clear signals that Sharif will be able “to push back against the rising tide of religious extremism.”

In a video released Tuesday, a Pakistani Taliban commander blamed Shiites for the unrest in Rawalpindi and called on Sunnis to “rise and kill the Shias, kill their officers and target their businesses.”

“I think it could definitely spiral out of control,” Thames said. “What is needed is just basic law enforcement, arresting people who kill others and incite violence, and that is not happening in any consistent way.”

Although much of the bloodshed can be traced to Sunni militant groups, Thames and other analysts said there is growing concern that the Shiite minority is also starting to organize militant groups.

On Jan. 2, two high-ranking officials of Ahl-i-Sunnat Wal ­Jamaat, a Sunni-dominated political group, were fatally shot in Islamabad by masked men on motorcycles. There was no assertion of responsibility, but at least a dozen other members of the group have been assassinated over the past year, according to the South Asia Terrorism Portal.

Haq Nawaz Khan in Peshawar, Nisar Mehdi in Karachi and Shaiq Hussain in Islamabad contributed to this report.