ORLANDO: Fifty people died and another 53 were injured early Sunday when a heavily-armed gunman opened fire and seized hostages at a gay nightclub in Orlando, Florida, police said, in the worst mass shooting in US history.

Terrified survivors -- who moments before were laughing and dancing with friends -- described how the gunman named by media as a US citizen of Afghan descent raked the club with bullets, prompting a police SWAT team to storm the venue.

“We have cleared the building, and it is with great sadness that I share we have not 20, but 50 casualties in addition to the shooter,” Mayor Buddy Dyer told a mid-morning news briefing in Orlando, more than doubling the previously given death toll.

“There are another 53 that are hospitalised.”

President Barack Obama was being kept up to date by his homeland security and counterterrorism aide on what Florida officials are investigating as a terrorist attack.

FBI official Ronald Hopper told reporters officials were “confident” there was no immediate further threat to the area, or to the United States.

Because of the scale of the crime, however, Orlando's mayor declared a city-wide state of emergency and has asked the Florida governor to take the same measure state-wide. The federal government has offered its full support with the probe.

Police have yet to officially identify the gunman but US television networks quoting law enforcement sources named him as Omar Mateen, who was born to Afghan parents in 1986 and lives in Port St Lucie, Florida, about two hours drive from Orlando.

CBS News reported that Mateen -- who died in a shootout with police after the siege -- has no apparent criminal history.

Hopper said authorities were looking into whether the suspect had “leanings” toward Islamic extremism, but Florida officials also invited a local Islamic leader to address the media in a bid to preempt a possible backlash against the Muslim community.

'Rescue hostages'

The chaotic events unfolded over a three hour period, beginning at around 2am when shots rang out amid the throbbing music at the Pulse Orlando nightclub near closing time.

Police said the shots were fired by a gunman armed with an assault rifle and a handgun.

A police officer working “extra duties” at the club responded, joined by two other officers, who engaged the suspect in a gun battle, Orlando police chief John Mina said.

“The suspect at some point went back inside the club where more shots were fired. This did turn into a hostage situation,” he told reporters.

“At approximately 0500 hours this morning, the decision was made to rescue hostages that were in there.”

Police then stormed the venue, using explosives and breaking through a wall with a wheeled armored vehicle known as a BearCat. Mina said about 30 people were rescued during the operation.

It was unclear whether all the victims were killed by the gunman or if some died in the ensuing shootout with police.

The attack coincides with gay pride month in the United States, with festive marches and events being held all over the country including in Orlando last week.

It was the second shooting incident in the city in just over 24 hours, coming shortly after singer Christina Grimmie was shot dead late Friday by a gunman who stormed a theater where she had just finished a gig. Mina said there was no indication of a link between the two shootings.

'We are in shock'

The father of the man named as the shooter in a massacre at a gay Florida nightclub says he's in shock and that he wasn't aware of anything his son might have been planning.

Mir Seddique is the father of Omar Mateen of Port St. Lucie, Florida.

Seddique told NBC News that his son got angry when he saw two men kissing in Miami a couple of months ago and thinks that may be related to the shooting.

“We are saying we are apologising for the whole incident. We are in shock like the whole country," said Seddique.

The father also says the incident has nothing to do with religion.

'Non-stop firing'

Speaking to Sky News, clubber Ricardo Negron, who was inside when the shooting began, described how the gunman raked the club with bullets.

“People just dropped on the floor. I guess the shooter was shooting at the ceiling because you could see all the glass from the lamps falling,” he told the network.

He described hearing “non-stop firing” which probably lasted less than a minute but felt like a lot longer.

“There was a brief pause in the shooting and some of us just got up and ran out the back.”

The Pulse club advertises itself online as “Orlando's hottest gay bar.” On its Facebook page, the club warned patrons: “Everyone get out Pulse and keep running.”

'Blood everywhere'

Witness Christopher Hanson said he heard “loud banging noises, like gunshots going off.”

“I didn't see any of the actual shooters. I just saw bodies going down and I was ordering a drink at the bar. I fell down. I crawled out. People were trying to escape out the back. I just know that when I hit the ground, I was crawling and I hit my elbows and my knees,” he told CNN.

He said there was “blood everywhere.” Rosie Feba was at the club with a friend when the shooting broke out.

“She told me someone was shooting. Everyone was getting on the floor,” Feba told the Orlando Sentinel.

“I told her I didn't think it was real, I thought it was just part of the music, until I saw fire coming out of his gun.”

Late Friday, a man thought to be a deranged fan fatally shot Christina Grimmie, a rising singing star and a former contestant on “The Voice”, as she was signing autographs after a concert in the central Florida city.

Orlando has a population of 270,930 and is the home of the famed Disney World amusement park and many other tourist attractions that attracted 62 million visitors in 2014.