US financier Jeffrey Epstein, awaiting trial on charges he trafficked underage girls for sex, was found dead in jail on Saturday of an apparent suicide, triggering outcry over how the high-profile detainee could die in custody.

The government and FBI immediately launched probes as politicians, law enforcement officials and alleged victims expressed shock that Epstein could take his own life, when a recent reported suicide attempt meant he should have been under close watch.

Epstein, a convicted pedophile who befriended numerous politicians and celebrities over the years, was found unresponsive in his cell around 6:30 am at the Metropolitan Correctional Center in New York from "an apparent suicide," the US Department of Justice said. 

He was pronounced dead at a hospital.

US Attorney General Bill Barr said he was "appalled" and instructed the Justice Department's inspector general to probe the circumstances. 

"Mr Epstein's death raises serious questions that must be answered," Barr said.

The FBI is also investigating, the Justice Department said.

The New York Times and other media reported Epstein hanged himself. The city medical examiner's office did not confirm the cause of death.

On July 23, the 66-year-old was found unconscious with marks on his neck. He was put on suicide watch for six days before being returned to his cell in a high-security part of the jail, the New York Times reported.

His death came one day after a New York court released a tranche of sealed legal documents, providing new details about what prosecutors allege was Epstein's sex-trafficking operation.

Epstein last appeared in court on July 31 when a judge told him that his trial wouldn't begin before next June.

The wealthy hedge fund manager had been charged with one count of sex trafficking of minors and one count of conspiracy to commit sex trafficking of minors. 

Epstein, who denied the charges, had faced up to 45 years in prison - effectively the rest of his life - if convicted. 

"We need answers. Lots of them," tweeted New York congresswoman Alexandria Ocasio-Cortez.

Republican Senator Ben Sasse, who heads the Senate's judiciary oversight committee, said the government had failed Epstein's alleged victims "yet again."

Epstein, whose friends included President Donald Trump, former president Bill Clinton and Britain's Prince Andrew, was convicted previously of paying young girls for sexual massages at his Palm Beach mansion.

They have all denied knowing anything about his alleged crimes.