Warning: this story contains a description of alleged sexual conduct.

Prosecutors in Chicago charged R&B singer R. Kelly on Thursday with 11 new sex-related counts involving one of the women who accused him of sexually abusing her when she was underage.

The new charges include counts that carry a maximum sentence of 30 years in prison, making them the most serious he faces.

The Grammy Award-winning singer was charged with four counts of aggravated criminal sexual assault, two counts of criminal sexual assault by force, two counts of aggravated criminal sexual abuse and three counts of aggravated criminal sexual abuse. The complainant was at least 13 and under 17 at the time.

The four aggravated criminal sexual assault counts carry maximum terms of 30 years in prison. Kelly is due in court to hear the new charges next Thursday.

Kelly was already facing 10 counts of aggravated sexual abuse involving four women over roughly a 10-year period that started in the late 1990s. Three of the women were minors when the alleged abuse occurred.

The new charges pertain to woman identified in the court filing by the initials J.P. Prosecutors didn't specify in the filing if she is the same person as one of the original four complainants who was also identified as J.P.

R. Kelly's lawyer, Steve Greenberg, told The Associated Press the 11 sex-related counts "are not really new."

"They are the same conduct, just charged differently," Greenberg said. "Same alleged victim, same timeframe, same facts."

Greenberg has long said Kelly expected to prevail at trial and in his comments Thursday, he said he expects the "same results."

Singer free since February arrest

Kelly pleaded not guilty to the original charges and denied any wrongdoing after his February arrest. He was freed when a suburban Chicago woman posted $100,000 bond, or 10 per cent of the $1-million US bond the judge set in his case. He briefly returned to police custody the following month for unpaid child support.

R. Kelly exits a Chicago lounge on Feb. 25. (Tyler LaRiviere/Chicago Sun-Times/Associated Press)

According to the new court filing, the first eight counts are from encounters that allegedly occurred between Jan. 1 and Jan. 31, 2010. Three others pertain to alleged encounters between May 1, 2009, and Jan. 31, 2010.

Among other things, prosecutors allege Kelly used force or threatened to do so to pressure the woman into sex or to perform oral sex on him. She was underage at the time, extending the statute of limitations for bringing charges to 20 years from her 18th birthday, they wrote.

Kelly doesn't face federal charges, but lawyer Michael Avenatti told the AP on Thursday that he expects the singer to face "serious federal charges" in the coming weeks.

Avenatti, who says he represents three Kelly victims, two parents and two whistleblowers, said he has provided "significant evidence" against Kelly to federal prosecutors in multiple U.S. attorneys' offices. He said he and his clients have been co-operating with prosecutors "for the better part of four months."

Attorney Michael Avenatti, who claims to represent several Kelly victims, has said he has provided 'significant evidence' to federal prosecutors. But the singer's lawyer, Steve Greenberg, said Avenatti isn't a credible source. (Kamil Krzaczynski/Reuters)

Greenberg said if Kelly ever does face federal charges, "we will deal with it." But he said Avenatti isn't a credible source for how the matter is unfolding.

"This man is accused of repeatedly stealing from his clients ... to fund an opulent lifestyle he hadn't earned," Greenberg said.

Avenatti, who said after Kelly's arrest that he had turned over video to prosecutors showing Kelly having sex with an underage girl, faces federal charges. They include ones accusing him of trying to shake down Nike and cheating his most famous client, porn star Stormy Daniels, out of $300,000 US.