At least 50 people were killed and 53 wounded in an attack Sunday on a gay nightclub in Florida, officials said, in the worst mass shooting in modern US history.

Mayor Buddy Dyer said the scale of the massacre at the Pulse nightclub in central Orlando was "unimaginable" and suggested the death toll could rise, as many of the injured were in critical condition.

Police chief John Mina said many bodies were still inside the building and it would "take some time" to clear the scene.

Dyer said at a press conference that many of the dead have yet to be identified. "There's blood everywhere," the mayor said.

Florida Governor Rick Scott declared a state of emergency for both the state and city.

"We're going to stand up to this terrorism," he told broadcaster CNN.

President Barack Obama was to deliver a statement at 1:30 pm (1730 GMT), the White House said.

Local and federal law enforcement officials said they are treating the attack as an act of terrorism but have yet to identify the gunman, who died in a shoot-out with police inside the club.

Mina said the shooter, who had an AR-15 type assault rifle, a handgun and an "unknown number of rounds," acted alone. There was no evidence that he had any outside help, he said.

US media, citing law enforcement sources, named the deceased shooter as Omar Mateen, a US citizen and Florida resident whose parents are of Afghan descent.

His father said in an interview with NBC News that the attack "has nothing to do with religion."

Mir Seddique told the broadcaster that his son had become angered after seeing two men kissing in Miami several months ago.

"We are saying we are apologizing for the whole incident. We weren't aware of any action he is taking. We are in shock like the whole country," he was quoted by the network as saying.

The assault on the nightclub - which the police chief described as "organized and well-prepared" - began at 2 am (0600) when a police officer posted outside the building confronted the shooter, who then opened fire.

The gunman then moved inside the club and took hostages.

"Everyone get out of Pulse and keep running," the club, which had been hosting a Latin-themed night, wrote on its Facebook page after the shooting began.

SWAT teams stormed the club three hours later, at 5 am, rescuing 30 people who were still inside, many of whom were hiding in a bathroom.

The shooter then died in a gunfight with officers.

Police said they have set up a crisis hotline and were asking for blood donations.

Nine officers were involved in the gun battle. One officer was injured in the eye, but not seriously, Mina said.

The head of the Human Rights Campaign, one of the country's leading organizations advocating for gay rights, said he was "devastated."

"We are grieving for the victims and our hearts are broken for their friends, families, and for the entire community," president Chad Griffin said. "This tragedy has occurred as our community celebrates pride, and now more than ever we must come together as a nation to affirm that love conquers hate."

Annual gay pride parades and festivals are commonly held in June across the United States.

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