Fifty dead, 53 injured in Florida mass shooting, says Orlando mayor

Nearly fifty people lost their lives and more than 53 were injured on Sunday as a heavily armed gunman, named by media as a US citizen of Afghan descent, opened fire and seized hostages at a crowded gay nightclub in Florida, in one of the worst mass shootings 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.

President Barack Obama was being kept up to date by his homeland security and counterterrorism aide on what Florida officials said was being investigated as a terrorist attack. The federal government has offered its full support with the probe.

Quoting law enforcement sources, US television networks named the shooter 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.

Police have yet to officially identify the gunman.

The chaotic events unfolded over a three hour period, beginning at around 2 AM 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, 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.