Rapper Travis Scott should have halted a show in Houston which saw a deadly crowd surge when he noticed people in trouble, the city's fire chief says.

"Everybody at that event had a responsibility," Samuel Peña told NBC. "From the artist on down."

Eight people were killed and hundreds more injured in a crush at his Astroworld concert in Texas on Friday.

The artist said he was "devastated" by the incident and was working to help victims' families.

His partner, the businesswoman and media personality Kylie Jenner, said they only learned there were fatalities after the event.

Among those injured is a nine-year-old boy said to be fighting for his life after he was trampled at the event.

"This young child and his family will face life-altering trauma from this day forward, a reality that nobody expects when they buy concert tickets," a lawyer for the family said in a statement on Tuesday.

Police said the victims were crushed in a surge of fans near the stage at around 21:30 local time.

Scott continued to perform for almost 40 minutes longer, eventually ending the set about 15 to 20 minutes ahead of the advertised time.

In footage from the concert, he can be seen interrupting his performance to ask for help for a fan who had passed out. But in another video, he asks: "Who asked you to stop? Y'all know what y'all came to do."

"At one point there was an ambulance that was trying to make its way through the crowd. The artist has command of that crowd," Mr Peña, the chief of the Houston Fire Department, told NBC's Today show.

"The artist, if he notices something that's going on, he can certainly pause that performance, turn on the lights and say: 'Hey, we're not going to continue until this thing is resolved'," he added.

In an Instagram video following the event, Scott said he was devastated about what had happened and was "sending prayers to the ones that were lost".

"My fans really mean the world to me," he said. "Any time I could make out anything that's going on, I just stopped the show and helped them get the help they need... I could just never imagine the severity of the situation."

Scott, one of the biggest names in rap music, launched Astroworld with concert promoters Live Nation in 2018.

On Monday, lawsuits were filed including allegations that Scott and surprise performer Drake incited the crowd at Friday's event, and that Live Nation failed to provide adequate safety measures.

Police have launched an investigation, including looking into separate reports that somebody in the audience was injecting people with drugs.

Several concertgoers had to be revived with anti-drug overdose medicine, including a security officer who police said appeared to have an injection mark in his neck.