A US-wide manhunt is underway for a cold-blooded killer who shot dead a total stranger on a street and upload it on Facebook.

The video shows suspect Steve Stephens walking up to an elderly man in the US city of Cleveland and shooting him at point-blank range while broadcasting the murder via his smartphone’s camera.

In the video Mr Stephens tells his followers: “I’m going to find somebody to kill. I’m going to kill this guy right here … this old dude,” before jumping out of the car and committing the alleged murder.

He approaches the elderly man who is walking on the footpath and asks him to say the name “Joy Lane”, reported to be Mr Stephens’ ex-partner.

“She’s the reason why all this is about to happen to you,” Mr Stephens says in the video. “How old are you?” he continues.

The victim, identified as 74-year-old Robert Goodwin Sr, shields his face with a shopping bag and pleads for his life before a gun is pointed at him and he is shot and killed.

“This is what we would consider a national search for Steve,” Cleveland Police Chief Calvin Williams told a news conference Tuesday morning (AEST). “So we are not going to leave any stone upturned.”

Mr Williams said authorities have yet to determine a motive for the shooting.

“We’ve interviewed several people involved, and I don’t think there is any rhyme or reason for this happening,” he said.

A reward of up to $50,000 has been offered for information leading to Mr Stephens’ arrest.

Police said the video was broadcast live on Facebook on Sunday local time, before the social media platform took it down approximately three hours after it was uploaded. Mr Stephens’ Facebook page has also been removed.

In a statement on Monday, police warned residents of nearby Pennsylvania, New York, Indiana and Michigan to be on alert.

In a separate video, the Cleveland man describes himself as a 37-year-old man that always had to “prove himself” that was always the “butt of peoples’ jokes”. 

He claims to have killed at least a dozen other people, but Cleveland Police say other homicides have not been verified.

The police advised Mr Stephens was armed and dangerous and asked the public to “not approach”.

The live streamed murder is the latest in a collection of horrific attacks filmed and uploaded via “live streaming” social media applications.

Chicago police charged and detained four people in January after an attack on a bound and gagged man. The incident, that police described as “sickening”, was broadcast live on Facebook.

In another separate incident, 18-year-old Marina Lonina of Columbus, Ohio faced multiple charges after she broadcast the rape of a 17-year-old friend last year via Periscope, another live streaming application.

Cybercrime expert Nigel Phair, at the University of Canberra’s Centre for Internet Safety, said offenders aimed for a kind of “quasi celebrity” status by filming and streaming their crimes.

Mr Phair said the disturbing content, posted and viewed by many, showed social norms in the real world were often abandoned online.

“There’s a disconnect…people aren’t seeing the consequences of their actions online,” he said.

A spokesperson for Facebook described the crime as “horrific”.

“We do not allow this kind of content on Facebook. We work hard to keep a safe environment on Facebook and are in touch with law enforcement in emergencies when there are direct threats to physical safety.”