At least 33 people have been killed in a powerful bomb blast at a Shia mosque in southern Pakistan, officials say.
Some 33 people were killed and many injured in the blast, Dawn reported citing the district health officer of Shikarpur, Munir Jokhio.
Police said at least 55 people were wounded in the attack after Friday prayers in the Shikarpur district of Sindh province.
The toll is expected to rise as many of the wounded are in a critical condition.
Local media reports suggest that the blast could have been a suicide attack, but police are investigating.
It is not yet clear who carried out the attack but Sunni militant groups have targeted the Shia minority in the past.
A number of people were trapped after the roof of the mosque collapsed due to the explosion, local media said.
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Prime Minister Nawaz Sharif, President Mamnoon Hussain, Pakistan Tehreek-i-Insaf chairman Imran Khan and Muttahida Qaumi Movement chief Altaf Hussain issued messages condemning the incident.
While describing the bomb strike a failure of the government, Majlis Wahdatul Muslimeen (MWM) announced a three-day mourning.
A bomb disposal squad has been dispatched to the area from Sukkur and rescue teams have reached the imambargah where the explosion took place.
The attack came as Sharif visited the city of Karachi, the capital of Sindh province.
It is the second major attack on an imambargah in the country since the beginning of 2015. The first was an attack on Rawalpindi's imambargah Aun Mohammad Rizvi in the garrison city's Chatian Hatian area.