America’s top law enforcer has slammed “serious irregularities” at the prison where billionaire Jeffrey Epstein was found dead.

US Attorney General William Barr said he was “appalled” and “angry” to learn of the New York jail’s “failure to secure the prisoner”.

It comes as stories emerge of overworked guards, staff shortages and prisoner overcrowding at the “gulag” which has been compared to facilities in Iran and Russia.

Two prison guards who were supposed to be checking on Epstein every 30 minutes reportedly failed to do so, with the union claiming they were both overworked.

One of the guards had worked five days’ overtime in a row and the other was also on over-time, BBC reports.

Epstein, who was meant to be sharing a cell, was also left on his own after the inmate was transferred.

“We are now learning of serious irregularities at this facility that are deeply concerning and demand a thorough investigation,” Mr Barr said.

Epstein was found dead on Saturday in an apparent suicide in his cell at the Metropolitan Correctional Centre (MCC) in lower Manhattan, where he was being held on new sex trafficking charges.

The prison was built in 1975 to house 474 inmates but reportedly contained 763 prisoners at the time of Epstein’s death.

The 12-storey complex in the city has detained such high-profile criminals as Mexican drug kingpin El Chapo, Gambino mafia crime boss John Gotti and World Trade Center bomber Ramzi Yousef.

Guards have been working up to 80 hours a week, according to the union representing prison guards – the American Federation of Government Employees Council of Prison Locals.

“The low pay, understaffing, augmentation, and mandatory overtime have created an environment inside our prisons where something like this is even possible,” said union president Eric Young.

A US professor, Jeanne Theoharis, who has written extensively about MCC, has described the jail as a gulag.

As conspiracy theories abound into Epstein’s death, US Attorney General William Barr said the investigation into the disgraced money manager’s sex crimes would continue.

“Any co-conspirators should not rest easy,” the top US law enforcement official said on Monday.

Mr Barr said the criminal case against Epstein was personally important to him and that the financier’s death denied his victims the chance to confront Epstein in a courtroom.

He did not outline what the jail’s “irregularities” were. The decision by officials at the Bureau of Prisons not to keep Epstein on a suicide watch has come under scrutiny.

Prior to his state conviction for sex crimes, Epstein had counted US President Donald Trump and former president Bill Clinton among his associates.

The New York City medical examiner said an autopsy had been completed on Epstein on Sunday but a determination on the cause of death is still pending.

Meanwhile two French ministers have called for an investigation in France into the activities of Jeffrey Epstein after links to the country were brought to light.

A French non-governmental organisation campaigning against child abuse alleged on Monday that Epstein shared ownership of a property in Paris and regularly spent time there.

“The American investigation has brought to light links with France,” Gender Equality Minister Marlene Schiappa and junior health minister Adrien Taquet said, without specifying what those links were.

“It appears to us essential, for the sake of the victims, that an investigation be opened in France to cast full light on the issue,” the ministers said.

Epstein was arrested on July 6 and pleaded not guilty to federal charges of sex trafficking involving dozens of underage girls as young as 14, from at least 2002 to 2005.

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-with AAP