50 dead at gay club in America's worst mass shooting

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 US.

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 TV 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.

It was 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 pre-empt a possible backlash against the Muslim community.

The chaotic events unfolded over a three-hour period, beginning at around 2 am (0600 GMT) 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 (0900 GMT) 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. Mina said about 30 people were rescued during the operation.