ISLAMABAD-  At least 54 people were killed, and more than 50 wounded, in an explosion that ripped through a crowded Shi'ite mosque in Pakistan today, underscoring a growing tide of sectarian violence in the South Asian nation.

Attacks against religious minorities have been on the rise in the country where radical Sunni Islamist groups often target mosques frequented by Shi'ites, whom they see as infidels.

Police said it was unclear what caused the explosion during Friday prayers at the mosque in the busy centre of Shikarpur, a city in Sindh.

"We are trying to ascertain the nature of the blast," said city police chief Saqib Ismail Memon. "A bomb disposal squad is examining the scene."

Part of the mosque collapsed after the explosion, burying some of the wounded under rubble. Bystanders pulled them out and piled them into cars for the journey to hospital.

"The death toll has now reached 49," Shaukat Memon, a doctor at a city hospital, told. More than 50 people were wounded, officials said.

Jundullah, a splinter group, which last year pledged support for the Islamic State group based in Syria and Iraq, claimed responsibility.

This is a developing story and will be updated accordingly