There were 13 workers in the mine at the time and nine bodies were recovered, said Adnan Farid, the area deputy commissioner.
The remaining four miners were rescued from the rubble and have suffered critical injuries, he said.
A government team from the mineral development department inspected the site of the incident and said the explosion took place "due to gas sparks inside the mine," Orakzai district police chief Nazeer Khan told Reuters.
A government report seen by Reuters said the blast caused the collapse of the mine, and that gas build-up had triggered the blast. It didn't specify what type of gas it was.
Coal deposits are found in the northwestern Orakzai district that sits on the Afghan border and mine accidents are common, mainly due to gas build-ups.
Mine workers have complained that a lack of safety gear and poor working conditions are the key causes of frequent accidents, labour union officials have said in the past.
(Reporting by Jibran Ahmad; Writing by Shivam Patel in New Delhi and Asif Shahzad in Islamabad; Editing by Andrew Heavens and Bernadette Baum)
By Jibran Ahmad
