A blast at a mosque in southwest Pakistan killed four people on Friday and wounded more than 20, police said, adding the death toll could rise.

The imam of the mosque, located 25 km (15 miles) from the city of Quetta, was killed in the explosion, police said.

“The blast was carried out through a timed device that was planted under the wooden chair of the prayer leader,” said Quetta’s chief of police Abdul Razzaq Chmeea.

Quetta is the provincial capital of the restive province of Balochistan where separatist insurgents and Islamist militants are active.

No group has claimed responsibility for the blast.

“The death toll could increase as some of the victims were seriously injured,” Shafqat Janjua, a police official, told Reuters.

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

Violence in Balochistan, which borders Afghanistan and Iran, has fuelled concerns about the security of projects such as a planned energy link from western China to Pakistan’s southern port of Gwadar.

Five people, including two policemen, were killed and 27 injured in a blast near a police station in Quetta in July, a week after a similar explosion killed two people.