WASHINGTON (AFP) - Former United States House speaker Dennis Hastert, once among America's most esteemed political figures, faced more public disgrace after prosecutors revealed sordid details of alleged sex abuse while he was a high-school teacher and wrestling coach.

Hastert allegedly molested boys as young as 14 and agreed to make secret payments totaling US$3.5 million (S$4.7 million) to one victim. It is a precipitous fall from grace for a man once second in line to the presidency.

On Saturday (April 9), Hastert's lawyer said his client acknowledged he had committed past "transgressions", but declined to describe those acts.

The statement came a day after federal prosecutors detailed for the first time allegations by five former high-school students of abuse that took place when Hastert taught and coached wrestling at Yorkville High School in Illinois from the 1960s to the early 1980s, before his congressional career.

Hastert's abuse consisted of "intentional touching of minors' groin area and genitals or oral sex with a minor", according to prosecutors, who condemned Hastert's "stunning hypocrisy".

The statutes of limitations have long passed on the alleged abuse, so Hastert is only being charged with breaking financial laws.

The 74-year-old Republican lobbyist pleaded guilty in October to illegally structuring bank withdrawals over four and a half years that prosecutors identified as hush money.

He told government agents he was being extorted by someone falsely accusing him of abuse, but recorded conversations between him and the person identified as "Individual A" cast doubt over his claim.

In the end, Hastert was able to actually pay Individual A a total of US$1.7 million - about half the agreed amount - between 2010 and 2014 in regular payments that only stopped when the government started investigating him.

At a sentencing hearing scheduled for April 27, prosecutors are expected to seek a six-month sentence for the Illinois lawmaker, taking note of his ill health as Hastert suffered a stroke after pleading guilty.

His lawyers asked for probation.

"Mr Hastert acknowledges that as a young man he committed transgressions for which he is profoundly sorry," defence attorney Thomas Green said in a statement.

"He earnestly apologises to his former students, family, friends, previous constituents and all others affected by the harm his actions have caused."

Hastert was speaker of the House of Representatives from 1999 to 2007. At the time, he was seen as a stolid, affable politician that helped the US legislature transition from the rancorous tenure of his predecessor Newt Gingrich.

Ironically it was another sex scandal that tainted Hastert's tenure as speaker.

An ethics committee found Hastert and other Republican leaders were "willfully ignorant" of multiple warnings about then lawmaker Mark Foley's inappropriate advances towards teenage congressional aides, known as pages.

Prosecutors said Hastert began paying Individual A after making the student athlete stay in his motel room during a wrestling camp and massaging his groin area.

"Defendant used his position of trust as a teacher and coach to touch a child's genitals and then undress and ask the child for a back massage in a motel room," prosecutors wrote of that encounter.

"There is no ambiguity; defendant sexually abused Individual A."

Officials said Hastert gave two other boys aged 14 and 17 a massage in the locker room before apparently performing an unspecified sex act on them in separate instances.

One of the boys told prosecutors that Hastert would sit in a recliner chair "in direct view of the shower stalls in the locker room" while the boys showered.

Mr Stephen Reinboldt, who died in 1995, was allegedly abused by Hastert throughout his time in high school from 1967 to 1971, according to his sister and others.

Prosecutors said that during a massage session, Hastert brushed his hand over the genitals of another boy, who found the encounter "very weird".

Hastert made the boys "feel alone, ashamed, guilty and devoid of dignity", prosecutors said.

"While the defendant achieved great success, reaping all the benefits that went with it, these boys struggled, and all are still struggling now with what defendant did to them."

On Friday, Hastert filed under a seal a response to the government's pre-sentence investigation report. A hearing has been set for Wednesday on whether his response can remain under seal.