50 dead in worst mass shooting in US history

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.

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

The shooter was identified as Omar Mateen, a man that a senior FBI official said might have had leanings towards Islamic State militants.

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

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.

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."

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.

"More likely than not it was an ideologically motivated attack," Florida Congressman Alan Grayson said.

"It might be that we've seen the commission of an awful hate crime."

Florida officials invited a local Islamic leader to address the media in a bid to pre-empt a possible backlash against the Muslim community.

ABC Australia