Fifty people have been killed, including the assailant, and at least 53 injured in an attack inside a gay nightclub in Orlando, in the US state of Florida, according to authorities.
Officials said on Sunday morning that the shooter, who was armed with assault-type rifle, handgun and some type of other device, died in a shootout with the police force at the Pulse nightclub.
US news networks have identified the gunman as a New York-born resident of Florida with Afghan origins. But investigators have yet to make an official announcement.
Ron Hopper, of the US Federal Bureau of Investigation, said authorities would not yet release the name of the shooter to avoid jeopardising the investigation. He added that it is too early to say if the incident is linked to “terrorism or hate crime”.
Orlando Police chief John Mina described the shooting as “one of the worst tragedies we have seen”, adding that police officers “were shaken by what they have seen inside the club”.
The injured had been transferred to nearby hospitals, police said.
“This did turn into a hostage situation. At approximately 05:00 hours (09:00 GMT) this morning, the decision was made to rescue hostages that were in there,” Mina said in an earlier press conference on Sunday.
In a statement, the White House press office said US President Barack Obama was briefed of the incident by Lisa Monaco, top advisor for Homeland Security and Counterterrorism.
Florida authorities said the attack was being treated as an “act of domestic or international terrorism”.
“We are investigating this from all parties’ perspective as an act of terrorism,” said Danny Banks, special agent in charge of the Florida Department of Law Enforcement.
“Whether that is domestic terrorist activity or an international one, that is something we will certainly get to the bottom of,” he added.
As the shooting occurred, the nightclub urged patrons to “get out” and “keep running” in a post on its Facebook page.
One witness, who said he was inside the building during the incident, said that he heard about 40 shots being fired.
Police said they carried out a “controlled explosion” at the club hours after the shooting broke out, while entering the nightclub.
Video posted online showed a large number of police and emergency vehicles outside the nightclub.
The nightclub shooting came just a day after a man thought to be a deranged fan fatally shot Christina Grimmie, a rising singing star who gained fame on YouTube and as a contestant on The Voice, while she was signing autographs after a concert in Orlando.