A fire engulfed an overcrowded block prison block in Indonesia's Banten Province in the early hours of Wednesday, killing at least 40 people and injuring dozens, a government spokesperson and media reports said.

The fire, which broke out at Tangerang Prison's Block C, was extinguished, yet authorities are still evacuating the facility, said Rika Aprianti, a spokesperson for the prison department at the Ministry of Law and Human Rights, according to Reuters.

"The cause is under investigation," she said.

The block housed inmates held for drug-related offenses and had a capacity for 122 people, she said. She did not say how many people were present when the fire broke out but confirmed that the jail was overcrowded.

The prison in Tangerang, which has a capacity of 600 people, housed more than 2,000 inmates, thus exceeding its capacity by far, according to government data as of September.

Kompas TV showed footage of firefighters trying to put out huge flames from the top of a building. The broadcaster reported the death of 41 people and that 8 were seriously injured.

"The initial suspicion is this was because of an electrical short circuit," Police Spokesperson Yusri Yunus told Metro TV, which cited a police report saying that 73 people also had light injuries.