An explosion at a plant dormitory has killed seven workers and injured one as of Wednesday noon in Baoding City, north China's Hebei Province, local authorities said.

Believed to be caused by illegally possessed explosives, the blast occurred at around 11:40 p.m. Tuesday in a privately-owned plant that produces slabstone in Niugang Village, Yixian County, the Baoding city government said.

Six workers were found dead in the collapsed building and another one died after being rushed to hospital, said Zhang Gangqiang, deputy director of the city government's information office.

It is unknown how many people were in the four-story dormitory building when the blast occurred, Zhang said.

The wounded worker was being treated in hospital.

Police are still investigating the blast.