Chicago police were questioning several individuals on Wednesday who allegedly gang raped a teenage girl and broadcast the assault on Facebook Live.

The victim was a 15-year-old girl who went missing on Sunday in the crime-plagued city in the US Midwest. She was found on Tuesday and taken to a children's hospital for treatment.

It was the latest in a series of troubling incidents in which violent acts have been streamed live on Facebook, including two fatal shootings and the kidnapping and torture of a disabled 18-year-old.

Authorities said on Wednesday that they were investigating the incident as a criminal sexual assault.

The live online video of the alleged gang rape attracted as many as 40 viewers at one time, but no one contacted police, according to reports. The video was later taken down.

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The Chicago Tribune newspaper reported that at least one juvenile had been taken into custody. Earlier, police said they were looking for as many as six suspects.

"It's disgusting. As a father with two daughters, I can't imagine what that mom is going through," Chicago's Police Chief Eddie Johnson said.

City councilor Michael Scott said investigators believe the girl knew the attackers and that they were all minors.

'Hideous crime'

Johnson became personally involved in the case after the girl's mother approached him on Monday with graphic photos taken from a Facebook Live video, showing several young men assaulting the teen, according to US media.

Police would not confirm details of the video, citing the ongoing investigation.

The social networking site declined to comment specifically on the incident, but said in a statement that the company took seriously its "responsibility to keep people safe on Facebook".

"Crimes like this are hideous and we do not allow that kind of content on Facebook," it said.

A man identified as the girl's uncle told US media that the young men involved were known in the local community.

"They are terrorising that entire neighbourhood," Reginald King told Chicago television station WGN.

Chicago sexual assault streamed live on Facebook

He said the group of teenagers number up to 35 to 50.

Chicago has been grappling with a surge in violent crime. Last year, the city endured its highest number of shootings and murders in nearly two decades.

Much of the violence is concentrated in a few impoverished areas, including the one where the teenage girl lives.