Bryan Singer. Picture: Reuters

Bryan Singer has been accused of sexually assaulting a 13-year-old boy.

The 53-year-old filmmaker allegedly behaved inappropriately towards Victor Valdovinos, who was a young teenager working as an extra on the set of the director's 1998 movie 'Apt Pupil'.

Victor told publication The Atlantic that he was told to undress and put on a towel for a locker room scene, and throughout the day the 53-year-old director would come and check on him.

He said: "Every time he had a chance - three times - he would go back there ... He was always touching my chest."

Victor alleged that, at one point, Singer - who was in his early 30s at the time - reached through the towel and "grabbed my genitals and started masturbating it".

He also accused the director of "rubbing his front part" on him, adding he "did it all with this smile".

He added: "I was frozen. Speechless. He came back to where I was in the locker room throughout the day to molest me."

The Atlantic expose also included allegations from three other anonymous accusers, who claimed to have had sexual encounters with the 'X-Men: First Class' filmmaker while in their teens.

One man claimed he was 15 when he slept with the director, while another stated he was 17 when he had sex with the then-31-year-old Singer and both insisted the filmmaker knew they were legally underage at the time.

The third man told the publication he had oral sex with the director when he was 17 or 18.

He said: "He would stick his hands down your pants without consent. He was predatory in that he would ply people with alcohol and drugs and then have sex with them."

The 'Usual Suspects' filmmaker's attorney, Andrew Brettler, denied his client ever had sex with underage boys and disputed the details of his accusers accounts.

He also claimed there appeared to be no record of Victor having been an extra on 'Apt Pupil' and questioned why he was unable to produce a pay stub or other documentation as proof he was there.

This isn't the first time the director - who was fired from 'Bohemian Rhapsody' two weeks before the end of production - has been accused of sexual misconduct in recent years.

In April 2014, Michael Egan accused him of raping him in 1999 when he was just 17, but the case was later dropped due to discrepancies in the alleged victim's account of what had happened.

And in December 2017, he was accused of sexual assault in relation to an alleged incident on board a yacht in 2003 by Cesar Sanchez-Guzman. Singer has denied the accusation but the case is still pending.