The human rights group Amnesty International urged Venezuelan authorities on Wednesday to determine whether troops used excessive force during a clash with prison inmates that left at least 58 dead.
The government said the violence in Uribana prison erupted last week when armed inmates fired at National Guard troops who were attempting to carry out an inspection. Nearly all those killed were prisoners.
"It's unacceptable that an inspection, which should be a routine operation, should lead to so much violence," Amnesty International said in a statement. "This clearly shows once again the crisis in the Venezuelan prison systems, which has reached truly alarming levels."
The group said Venezuela should conduct "an independent, objective and impartial investigation" to find out who was responsible for the slayings.
The chief prosecutor has begun an investigation, as has the National Assembly.
Rupert Colville, a spokesman for the office of the U.N. High Commissioner for Human Rights, also expressed concern on Tuesday about "an alarming pattern of violence in Venezuelan prisons, which is a direct consequence of poor conditions."
Venezuelan Foreign Minister Elias Jaua called the appraisal by the U.N. human rights office "hasty." He said on television Wednesday that the government will provide the U.N. agency with all relevant information, and insisted that the government has been complying with recommendations made by U.N. officials in an annual review of prisons.
"It's part of a complex battle, a difficult battle, but it's being confronted every day," Jaua said.
Penitentiary Service Minister Iris Varela renewed pledges to build more prisons.
Critics and government opponents say that despite previous pledges the government has failed to build more prisons or get a grip on rampant violence in the country's overcrowded penitentiaries.