A blast at a mosque in Quetta in southwestern Pakistan on Friday killed at least 13 people, including a senior police officer, and injured 20 others, police said.

Police said an improvised explosive device had been planted at the mosque inside a seminary in the city, the capital of Balochistan province, which borders Afghanistan and Iran.

It exploded during evening prayers on Friday, the second bombing in Quetta this week.

"So far, 13 bodies and 20 injured have been brought here," Dr. Wasim Baig, spokesperson for a local hospital told Reuters, adding that many of the injured were in serious condition.

A senior police officer, Haji Amanullah, was among the dead. His son was killed by unknown persons last month.

The attack was not immediately claimed by a specific group.

A spokesperson for Pakistan military said paramilitary troops were conducting a search operation around the site.

Mineral- and gas-rich Balochistan is at the centre of the $60 billion US China Pakistan Economic Corridor, which is part of China's Belt and Road infrastructure project.

Violence in Balochistan has fuelled concerns about the security of projects such as a planned energy link from western China to Pakistan's southern port of Gwadar.

The attack came just days after a roadside bomb in Quetta hit a paramilitary force vehicle, killing two troops.

Hizbul Ahrar, an offshoot of the Pakistani Taliban, claimed responsibility for Tuesday's attack.