Thirty-seven prison inmates have been killed during an overnight raid by government security forces in Venezuela's southern jungle state of Amazonas, the governor said on Wednesday.

Human rights advocates have long complained that violent gangs exercise de facto control over many of the chaotic country's prisons and have ready access to automatic weapons and even hand grenades.

The government seeks to reassert control over understaffed prison facilities by sending in special forces from time to time, but deadly confrontations often ensue.

"There was a massacre," tweeted Liborio Guarulla, the Amazonas governor, who said 37 prisoners had been killed.

"The morgue is totally overwhelmed," he said later in an interview, adding that the raid on the prison in the state capital Puerto Ayacucho started around midnight.

Venezuela's state prosecutor's office said later on Wednesday that it was investigating 37 deaths in Amazonas. It added 14 officials had been injured