The UN has issued a withering appraisal of the human rights situation in Venezuela, as horrific details emerged of the injuries inflicted on a navy captain allegedly tortured to death during a crackdown on alleged plotters against Nicolás Maduro.

A report by UN human rights chief Michelle Bachelet – which follows a three-day mission to the South American country last month – accuses Maduro’s security forces of committing a series of “gross violations” against Venezuelan dissenters and urges him to disband a notorious special forces group blamed for a wave of politically-motivated killings.

The 16-page document also contains chilling details of the techniques allegedly being used by Venezuela’s security and intelligence services to interrogate and intimidate members of the political opposition, as Maduro battles to retain power.

The report says that in scores of cases identified by researchers from the Office of the United Nations High Commissioner for Human Rights (OHCHR) “women and men were subjected to one or more forms of torture or cruel, inhuman or degrading treatment or punishment, including electric shocks, suffocation with plastic bags, water boarding, beatings, sexual violence, water and food deprivation, stress positions and exposure to extreme temperatures”.

It adds: “Security forces and intelligence services routinely resort to such practices to extract information and confessions, intimidate, and punish the detainees.”

The report is also highly critical of FAES, a shadowy special forces unit activists suspect has been tasked with subduing opposition to Maduro in Venezuela’s impoverished periphery.

The report said Bachelet’s team had interviewed relatives of 20 young men killed by FAES between June 2018 and April this year, all of whom described a similar modus operandi. Heavily-armed, balaclava-wearing agents would storm homes and separate their targets from their families before shooting them.

“According to their relatives, almost all of the victims had one or more shots in the chest,” the report said.

Maduro’s government rejected the “distorted” report in an 11-page rebuttal that accused Bachelet of offering “a selective and openly biased vision” of human rights in Venezuela.

It said: “The imprecisions, mistakes, decontextualizations and false claims are innumerable.”

The UN report was released amid growing outrage over last Friday’s killing of Rafael Acosta, a Venezuelan navy captain who had been detained during a roundup of supposed anti-Maduro conspirators.

Acosta’s wife has claimed he was tortured to death by military counter-intelligence officers – claims supported by a copy of the homicide squad’s preliminary autopsy that was leaked to a pro-government newspaper on Wednesday.

The newspaper said pathologists had given the cause of death as “severe cerebral edema [brain swelling] caused by acute respiratory failure caused by a pulmonary embolism caused by rhabdomyolysis [a potentially life-threatening breakdown of muscle fibers] by multiple trauma”.

Doctors reportedly identified internal bleeding in Acosta’s digestive tract, colon and neck as well as a pulmonary embolism and broken ribs.

A separate report claimed Acosta had suffered 16 broken ribs and a whiplash-like back injury during the alleged torture session.

At a press conference in Caracas on Thursday, opposition lawmaker Delsa Solorzano said the autopsy left in no doubt what had happened.

“There are lots of technical words that those of us who aren’t doctors can’t fully understand,” Solorzano told reporters. “But at the end of day what they mean is they beat him to death.”

Venezuelan authorities have moved quickly to portray the killing as the act of a small group of intelligence officers, acting independently.

On Monday attorney general Tarek Saab – a close Maduro confidant – said he had ordered two agents suspected of involvement in the “unfortunate incident” to be arrested on manslaughter charges.

Saab promised an “objective, independent and impartial” investigation and said the culprits would receive “exemplary punishment”.

Bachelet’s report suggested neither was likely to happen.

It said: “The attorney-general’s office has regularly failed to comply with its obligation to investigate and prosecute perpetrators.”