Gunmen in north-west Nigeria have stormed a mosque, killing Muslim worshippers in what the local government has called a "reprisal attack".

Police in Katsina state said 17 worshippers were shot dead by the assailants - known locally as bandits - but other reports from the region say the death toll is higher.

The mosque, located in the village of Unguwan Mantau, was attacked on Tuesday because local residents had ambushed and killed a number of "bandits" two days earlier, Katsina state official Nasir Muazu said.

Banditry is rife in north-western Nigeria, where armed gangs target locals, often for financial gain.

Muazu, Katsina state's commissioner for internal security, said the attack occurred during morning prayers and that the gunmen shot "sporadically".

As of Wednesday afternoon, the attackers remained at large. Muazu said the police and military personnel had been deployed to find them.

Despite the reprisal, Muazu said the local government "reaffirms its unwavering support for community-based security initiatives".

Residents of Unguwan Mantau, meanwhile, have been mourning and tending to the wounded. As per tradition, grieving families have been sitting outside their homes, accepting condolences from visitors.

Ali Isa Pantami, a popular Islamic cleric and Nigeria's former communications minister, hit out at the killers, saying they "betray the sense of humanity on earth".

"We urge the relevant security agencies to ensure the arrest of the criminals and bring an end to this inhumanity and criminality of the highest order," he wrote in a statement posted to social media.

"The killers of defenceless people are heartless, malevolent, and worse than animals."

Nigeria's vast north-west region borders Niger and criminal gangs criss-cross between the two countries, evading capture.

Katsina and neighbouring state Zamfara have been hit the hardest by banditry in the region, losing the most lives and experiencing the most displacement as a result of the problem.

Just days ago, kidnappers in Zamfara abducted and killed at least 35 people, despite ransoms being paid for their release, a local official told the BBC.