Kim Foxx said she was “sickened” by allegations made in a documentary about the singer, which he denies.
An Illinois prosecutor has urged alleged victims of abuse by R Kelly to come forward.
The call, by illonois prosecutor Kim Foxx, follows the release of Surviving R Kelly, a documentary detailing allegations of abuse, predatory behaviour and paedophilia that have followed the singer for years.
Authorities say they have been inundated with calls following the documentary, which aired in the US last week.
Mr Kelly has vehemently denied allegations against him.
Speaking on Tuesday, Ms Foxx said her office had spoken with two families who claim to have lost contact with relatives who spent time with the singer.
“Please come forward,” she said, addressing possible victims of Mr Kelly’s alleged abuse. “We cannot seek justice without you.”
“There’s nothing that can be done to investigate these allegations without the cooperation of both victims and witnesses,” she said, explaining that there is no active investigation and that launching one would require the co-operation of alleged victims and witnesses.
Ms Foxx added it took courage to re-experience trauma by reporting assault and domestic violence, but said dedicated professionals were on hand to work with alleged victims should they come forward.
“This isn’t one of those situations where it’s just forensics,” she said. “We need actual witnesses and victims.”
Mr Kelly’s lawyers have vehemently denied all allegations against him, and accused Ms Foxx of seeking publicity.
Steve Greenberg said in an interview that the accusations were “another round of stories” used to fill “reality TV time”.
Ms Foxx called last week’s six-part documentary “deeply, deeply disturbing” and said she was “sickened” by the allegations within it.
The show contained the latest of a series of allegations that have followed Kelly through his career.
In 2008 a jury acquitted the singer of 14 counts of child pornography. Prosecutors argued a video depicted sex acts between him and a 13-year-old girl, but both Kelly and his alleged victim denied it was them in the film.
Later, Kelly was forced to “unequivocally” deny claims made in a Buzzfeed investigation that he held women in a cult-like environment.
Former members of Kelly’s inner circle told the website he dictated what a group of women “eat, how they dress, when they bathe, when they sleep, and how they engage in sexual encounters that he records”.
The article alleged the singer confiscated the women’s phones and enforced strict rules for his so-called “babies”.
Parents of one woman said to be living with the 50-year-old claimed their daughter is being held against her will.
All the women mentioned were over 18 and the singer has not been charged with any crime in relation to the article.
Kelly has denied all allegations against him.
Last year he addressed accusations in a 19-minute song titled I Admit, refuting claims as he sang: “I admit I f*** with all the ladies, that’s both older and young ladies.
“But tell me how they call it paedophile because of that, s**t is crazy.”