At least 50 people have been killed and another 53 injured after a heavily armed gunman opened fire and seized hostages at a gay nightclub in Orlando, Florida, police say, in the worst mass shooting in US history.

Key points: President Barack Obama describes the shooting as "an act of terror"

State of emergency declared across Orlando

Gunman was killed during shootout with police

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Terrified survivors — who moments before were laughing and dancing with friends — described how the gunman raked the Pulse nightclub 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.

He said 39 people died inside the club, two outside, and nine others died after being rushed to hospital.

The city of Orlando has begun releasing names of the victims.

The shooter was identified as Omar Mateen, 29, who called 911 on Sunday morning (local time) and made comments saying he supported the Islamic State (IS) militant group, officials said.

But US officials cautioned they had no conclusive evidence of any direct connection with IS or any other foreign extremist group.

Mateen was born to Afghan parents in 1986 and lived in Port St Lucie, Florida, about two hours' drive from Orlando.

He worked as an armed guard for security firm G4S and was carrying an AR-15-type semi-automatic rifle and a handgun when he walked into the Pulse nightclub and opened fire.

The suspect's father told NBC news his son may have been motivated by homophobia and not by his Muslim faith.

"This had nothing to do with religion," Mir Seddique told the network, recalling a recent incident in downtown Miami.

"He saw two men kissing each other in front of his wife and kid and he got very angry," the father said, apologising to the victims.

His former wife, who left him in 2011 fearing for her life, said he was a violently abusive man who wanted to be a police officer.

US President Barack Obama expressed heartbreak at the "horrific massacre", branding it an act of terror and hate.

"Although it's still early in the investigation, we know enough to say that this was an act of terror and an act of hate," Mr Obama said, later ordering flags at half-staff as an act of mourning.

On gun violence Mr Obama said: "To actively do nothing is a decision as well."

Head of prominent US Muslim advocacy group Sunday condemned the massacre as a "hate crime".

"It violates our principles as Americans and as Muslims," he said. "Let me be clear, we have no tolerance for extremism of any kind."

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

30 hostages rescued: police

Worst mass shootings in US history: Orlando, Florida, June 12, 2016: 49 people die, another 53 are injured when a heavily-armed gunman opens fire and seizes hostages at a gay nightclub

Blacksburg, Virginia, April 16, 2007: 23-year-old student of Korean origin goes on a rampage at Virginia Tech University, killing 32 people

Newtown, Connecticut, December 14 2012: A young man kills 26 people, including 20 children at Sandy Hook elementary school. He also fatally shoots his mother. He kills himself

Killeen, Texas, October 16, 1991: A man shoots dead 22 people in a restaurant and then kills himself. Another injured victim dies later



Other significant gun attacks:

Littleton, Colorado, April 20, 1999: Two teenage boys shoot and kill 12 classmates and a teacher at Columbine High School before killing themselves

Aurora, Colorado, July 20, 2012: A man kills 12 people and injures 70 more when he opens fire at a movie theatre showing a late-night premiere of a Batman film

Fort Hood, Texas, November 5, 2009. US army psychiatrist Major Nidal Hasan opens fire at his military base, killing 13 people and wounding 42

The chaotic events unfolded over a three-hour period, beginning about 2:00am (local time) 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 said.

"At approximately [5:00am] 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 armoured vehicle known as a BearCat.

Mr 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 theatre where she had just finished a gig.

Mr Mina said there was no indication of a link between the two shootings.

'Non-stop firing': Witnesses describe shootings

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 said.

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."

A post on the club's official Facebook page uploaded during the initial stages of the shooting read: "Everyone get out of Pulse and keep running."

Christopher Hansen was inside the nightclub when the shooting began.

"I was hearing the 'bang, bang, bang', I thought it was part of a song … then when you would turn around the person next to you is screaming, there was blood splattering, I didn't know it was mine or someone else's and when I got out I realised it wasn't mine, it was somebody else's," he told reporters.

The mother of hostage Eddie Justice spoke with reporters outside the club, saying she was speaking with her son on the phone as he huddled in a bathroom.

"He was shot?" the mother was asked.

"We don't know. He was in the bathroom with a hostage. He's a hostage from the shooter," the mother said.

"He was texting me and calling me.

"[He told me] the shooter has them in the women's bathroom and he was going to die.

"I made him get off the phone because I had the dispatcher on the one phone and him on the other."

Mr Justice was later confirmed as being among the victims.

AFP/Reuters