Multiple people were wounded in a shooting on Wednesday at a US Immigration and Customs Enforcement (ICE) detention facility in the US city of Dallas, Homeland Security Secretary Kristi Noem said.

"Details are still emerging but we can confirm there were multiple injuries and fatalities," Noem said in a post on X.  "The shooter is deceased by a self-inflicted gun shot wound."

"While we don't know motive yet, we know that our ICE law enforcement is facing unprecedented violence against them," Noem added. "It must stop."

Acting ICE director Todd Lyons, in an interview with CNN, said the attack may have been carried out by a sniper.

"Preliminary information is a possible sniper," Lyons said. "We have three individuals that are down at this time. We're not sure on their condition. They've been taken to the hospital."

Local television station Fox4 said the three people shot were detainees in ICE custody.

It said the gunman was a white man on the roof of a building who shot himself as law enforcement agents approached.