A gunman with possible "leanings toward radical Islamic terrorism" opened fire inside a crowded gay nightclub in Orlando, killing 50 people and injuring dozens more in a rampage that officials said was the deadliest mass shooting in US history.

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Suspected gay nightclub mass shooter Omar Mateen.Photo:

Authorities were investigating it as an act of terrorism.

At least 53 other people were hospitalized, most in critical condition, officials said. A surgeon at Orlando Regional Medical Center said the death toll was likely to climb.

Mayor Buddy Dyer said all of the dead were killed with the assault rifle.

"There's blood everywhere," Dyer said.

The suspect exchanged gunfire with an officer working at the gay club known as Pulse around 2 a.m., when more than 300 people were inside. The gunman then went back inside and took hostages, Police Chief John Mina said.

Around 5 a.m., authorities sent in a SWAT team to rescue the hostages.

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An injured woman receives medical attention after the nightclub shooting.Photo: EPA

Jackie Smith, who was inside the club, said two friends next to her were shot.

"Some guy walked in and started shooting everybody. He had an automatic rifle, so nobody stood a chance," Smith said. "I just tried to get out of there."

In addition to the guns, the shooter also had some sort of "suspicious device," Mina said.

Authorities were looking into whether the attack was an act of domestic or international terror, and if the shooter acted alone, according to Danny Banks, an agent with the Florida Department of Law Enforcement.

"This is an incident, as I see it, that we certainly classify as domestic terror incident," Orange County Sheriff Jerry Demings said.