Update as of 7:10 a.m. EST: Jundallah, a Sunni militant organization, which had pledged allegiance to the Islamic State group in November, has claimed responsibility for the attack on a Shia building in Pakistan’s Sindh province, according to a Reuters report.

“Our target was the Shia community ... they are our enemies,” Fahad Marwat, a spokesperson for the group that was previously linked to the Pakistani Taliban, reportedly said.

Meanwhile, the number of people killed in the blast has risen to 33, according to a report by Dawn. Fifty-five people have also been injured in the attack and are being treated at a hospital in the city of Shikarpur.

Original story:

At least 20 people were killed and 50 others were injured in a bomb blast inside a Shia congregation hall in Pakistan's southern province of Sindh on Friday, according to local media reports. This is the second incident targeting a Shia building in the last one month in the Sunni-dominated country.

The blast reportedly took place inside the Karbala Maula Imambargah, located in the Lakhi Dar area of Shikarpur district, during Friday prayers. A number of people are still believed to be trapped inside the debris after the roof of the building collapsed, according to a report by Dawn, a Pakistani newspaper. The death toll is expected to rise further as many of the wounded are in a critical condition.

Although it is not yet clear who carried out the blast, which, according to local reports, could have been a suicide attack, militant groups, including al Qaeda and the Taliban, have targeted Shia buildings in the past. On Jan. 9, a powerful blast in an imambargah -- a place of congregation for Shia Muslims -- in the city of Rawalpindi had led to the deaths of eight people.

Following Friday's blast, the provincial government in Sindh declared a state of emergency in the region and ordered an investigation into the incident, according to a report by Express Tribune, a Pakistani daily. Officials also denied allegations that the attack had taken place because of a lapse in security.

“The entire Pakistan is under threat and militants attack wherever they find a soft target but the blast did not occur because of a security lapse,” Sharjeel Inam Memon, the information minister in the Sindh government, reportedly said.

Shias, who constitute nearly 20 percent of Pakistan’s population, face intense persecution not only from militants of al Qaeda and Tehreek-e-Taliban Pakistan, but also from local militant groups such as Lashkar-e-Jhangvi, which has, on several occasions, called them “the greatest infidels on earth.”

According to figures released by the South Asia Terrorism Portal -- a website focused on militancy in South Asia -- nearly 4,000 people, mostly Shias, have been killed in over 2,600 sectarian attacks across Pakistan since 1989.