AceShowbiz - R. Kelly's sex crime accusers are reportedly set to testify before a federal grand jury in Chicago, Illinois this week (end June 2).

The "I Believe I Can Fly" hitmaker has pleaded not guilty to 10 felony aggravated sex abuse charges for allegedly assaulting four victims, three of whom were underage at the time. He is also facing the federal grand jury probe.

Sources connected to the investigation tell U.S. website The Blast, that witnesses arrive in the city on Tuesday, May 28 to give testimony to the grand jury. Questioning will reportedly focus on allegations Kelly and his entourage transported underage girls across the U.S., with a view to bringing federal charges of sex trafficking and child exploitation.

Kelly was charged with the 10 abuse felonies in February, but officials are reportedly close to filing indictments against the singer in the Southern District of New York and the Northern District of Illinois, and plan to rely on the grand jury testimony.

According to The Blast, plane tickets, text messages, and receipts which allegedly prove his team bought flights for underage girls, have been collected as evidence. Investigators from New York also travelled to Atlanta, Georgia to collect testimony for the federal case.

The Blast's sources believe indictments will be handed out shortly after his accusers have testified in Chicago.

The 52-year-old has been dogged by allegations of underage sex for years and was acquitted on child pornography charges in 2008. However, he came under renewed scrutiny in 2017 after BuzzFeed News published parents' allegations he had "brainwashed" their daughters into a "sex cult".

More women, including his ex-wife Andrea Kelly, have since accused him of abusive behaviour and took part in an investigative documentary series, "Surviving R. Kelly", which aired in January. He denies all the charges and sexual abuse allegations.