An estimated 50 people were killed at a gay nightclub after a heavily armed gunman seized hostages early Sunday, prompting a police SWAT team to storm the venue, officials said.

“Unfortunately, there are people who died from gunshot wounds, maybe around 50, inside the night club,” FBI special agent Ron Harper told a media briefing.

He said around 53 injured were taken to three area hospitals.

“This did turn into a hostage situation,” Orlando police chief John Mina said. “At approximately 0500 hours (0900 GMT) this morning, the decision was made to rescue hostages that were in there.”

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

Police initially said about 20 people had been killed in the attack, with 42 injured, but later revised the death toll up to 50, with 53 hospitalised, including one police officer who had been shot in the head but whose “life had been saved by his Kevlar helmet”. They confirmed the gunman was armed with an assault rifle, handgun and “some kind of device”.