A bomb blast ripped through a Shiite mosque in southern Pakistan as worshippers were gathering for Friday prayers, killing 56 people and wounding dozens more, officials said.
The blast was one of the deadliest sectarian attacks to hit the country in years and comes as Pakistan is already struggling to contain a surging militancy following the horrific Peshawar school attack that killed 150 people, most of them children, in December.
This area of Pakistan has largely been spared the intense attacks and violence seen over the years in the northwestern tribal areas bordering Afghanistan and also the port city of Karachi, indicating the country's terrorism challenges could be extending into new territory.
Hadi Bakhsh Zardari, the deputy commissioner of Shikarpur district where the blast hit, gave the death toll and said 31 wounded are still in area hospitals.
In a sign of how serious the explosion was, Dr. Shaukat Ali Memon, who heads the hospital in Shikarpur, appealed on Pakistan's state television for residents to donate blood for the wounded.
Pakistani television showed area residents and worshippers frantically ferrying the dead and wounded to the hospital. Local media reported that parts of the roof had collapsed on the worshippers, and some people had been trapped inside.
Prime Minister Nawaz Sharif said in a statement condemning the violence that it was a suicide bombing but Zardari and other officials said they were still investigating the cause.
"Explosive experts and police are still debating whether it was a planted bomb or a suicide attack due to conflicting evidence on either side," Zardari said.
Fahad Mahsud, a spokesman for the Sunni militant group Jundullah, claimed responsibility for the attack in a telephone call to The Associated Press, but gave no details about how it was carried out. The militant group has previously claimed responsibility for attacks on Shiites and other religious minorities, including an attack on a church in the northwestern city of Peshawar in 2013 that killed 85 people.
Many Sunni extremists do not consider Shiites to be true Muslims. Sunni militants in Pakistan have bombed Shiite mosques, killed Shiite pilgrims traveling back and forth to neighboring Iran and assassinated prominent Shiite religious figures or community leaders.
While Karachi has been the site of repeated bombings blamed on militant groups such as the Pakistani Taliban, the northern part of Sindh province has generally been much more peaceful.
But recent years have seen a trend of extremist organizations becoming increasingly active in the central and northern part of the province, according to a new report by the United States Institute of Peace.
Friday's attack in Shikarpur comes as Pakistan is already struggling to address militancy following the Peshawar attack that shocked the country. The military has stepped up campaigns in the northwest, and the prime minister has lifted a moratorium on the death penalty in terrorism-related cases.