Houston fire chief Samuel Pena said the incident began at around 9 pm when "the crowd began to compress towards the front of the stage".

"That caused some panic and it started causing some injuries, people began to fall out, become unconscious and it created additional panic," Pena said at a press conference outside NRG Park.

"We had at least eight confirmed fatalities tonight and scores of individuals that were injured," he said, adding the cause of death cannot be confirmed until after medical exams are completed.

"We transported 17 patients to the hospital... 11 of those that were transported were in cardiac arrest," he said.

Around 50,000 people were in the crowd at rapper Travis Scott's Astroworld Festival, Pena added.

Houston police said they were investigating the cause of the mass casualty incident and looking at video footage from the arena.

"Over the course of just a few minutes, suddenly we had several people down on the ground experience some type of cardiac arrest," police said, adding the concert was ended after the incident occurred.

Astroworld is a music festival created by American rapper Travis Scott and launched in 2018.

The 29-year-old rapper, who has a child with Kylie Jenner, made his breakthrough in 2013 and has had six Grammy nominations.