Image copyright Chicago Police Image caption All four suspects have been charged with unlawful restraint and aggravated battery.

President Barack Obama has described the torture of a mentally disabled man in Chicago, streamed on Facebook Live, as a "despicable" hate crime.

The president said that while he did not believe US race relations had worsened, public exposure to race crimes had grown in the digital age.

He said the video encapsulated problems that have existed for some time.

Three teenagers and a woman, 24, are due in court on Friday accused of aggravated battery with a weapon.

Jordan Hill, Brittany Covington and Tesfaye Cooper, all 18, and Tanishia Covington are also accused of aggravated unlawful restraint.

Torture video prompts online race fight

Image copyright EPA Image caption The president said 'the overall trajectory' of race relations in the US 'is actually very positive'

In the video, the assailants can be heard making derogatory statements against white people and Donald Trump.

"What we have seen as surfacing, I think, are a lot of problems that have been there a long time," President Obama told CBS Chicago.

"Whether it's tensions between police and communities, hate crimes of the despicable sort that has just now recently surfaced on Facebook."

The president said that while the Chicago incident was grounds for serious concern, he remained optimistic about the long-term state of US race relations.

"The good news is that the next generation that's coming behind us… have smarter, better, more thoughtful attitudes about race," he said.

"I think the overall trajectory of race relations in this country is actually very positive. It doesn't mean that all racial problems have gone away. It means that we have the capacity to get better."

Media playback is unsupported on your device Media caption Chicago live-streamed attack: 'Don't blame Black Lives Matter'

All the suspects, apart from Tanishia Covington, each face an additional count of residential burglary.

Mr Hill is further charged with robbery and possession of a stolen motor.

Image copyright Facebook Image caption The victim was made to drink from a toilet bowl and had a gag placed over his mouth

Police said the unnamed 18-year-old victim was dropped off by his parents at a McDonald's restaurant on 31 December to meet his friend, Jordan Hill, who later became one of his attackers.

The pair drove around for two days, sleeping inside a van that Mr Hill had allegedly stolen without the victim's knowledge, before ending up at the Covington sisters' home.

Police said a play fight led to a prolonged assault in a flat on the Illinois city's west side.

The captive was made to drink from a toilet bowl, had part of his scalp removed with a knife, and was bound, gagged and beaten.

Media playback is unsupported on your device Media caption Supt Eddie Johnson: "What would make individuals treat somebody like that?"

At one point in the 30-minute video, the attackers can also be seen cutting the victim's clothes, dropping cigarette ash on him and pushing his head back with a foot.

Chicago Police Commander Kevin Duffin said the victim finally escaped when his attackers went downstairs to confront a resident who had called police because of the noise.

Cdr Duffin said the racial slurs and references to the victim's mental capacity, depicted in the video, led to the hate crime charges.

The escaped captive was found disorientated and traumatised, walking the streets in shorts and sandals with Mr Hill.

The victim, who had difficulty communicating with police, was taken to hospital for medical treatment, and discharged.

More on violence in Chicago

Media playback is unsupported on your device Media caption Life and death on the lost streets of Chicago

In other videos posted online the young man is forced at knife-point to say: "I love black people".

The incident has provoked a strong reaction on Twitter, especially among the alt-right - the fringe group that celebrated US President-elect Trump's election win with Nazi salutes.