At least 40 people have been killed and more than 50 wounded, in an explosion that ripped through a crowded Shi'ite mosque in Pakistan, underscoring a growing tide of sectarian violence.

Attacks against religious minorities have been on the rise in Pakistan, 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 Pakistan's southern province of 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.

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

"Our target was the Shia community (mosque)... They are our enemies," said Fahad Marwat, a Jundullah spokesman. He did not elaborate.

Sain Rakhio Merani, a regional police official, said the blast was probably caused by an explosive device, although Pakistani television quoted some residents as saying they saw a man wearing a suicide vest.

The attack came as Pakistan adopts new measures to tackle Islamist extremists following a massacre of 134 children last month at an army-run school in the volatile northwestern city of Peshawar.

The government has pledged to crack down on all militant groups, reintroduce the death penalty, set up military courts to speed convictions and widen its military campaign in the lawless tribal areas.

Yet Pakistan's religious minorities, among them Ahmadis, Christians and Hindus, say the government is doing little to alleviate their daily struggle against humiliation, discrimination and often violence.

Shi'ites make up about a fifth of Pakistan's population of around 180 million. More than 800 Shi'ites have been killed in attacks since the beginning of 2012, according to Human Rights Watch.