A mass shooting at a nightclub in Orlando, Florida, has left an estimated 20 people killed after a heavily-armed gunman seized hostages early Sunday.

"Unfortunately there are people who died from gunshot wounds, maybe around 20, inside the night club," FBI special agent Ron Harper told a media briefing.

He said around 42 injured were taken to three area hospitals.

The shooting turned into a hostage situation, prompting a police SWAT team to storm the venue, 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," he said.

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

The gunman took hostages at the Pulse Club and was later killed by police. A controlled explosion was reported at the venue.

Video footage shared on social media showed a high police presence around the club.

A paramedic said the casualties will rise as more people are being brought out of the club.

A witness heard gunfire from a vehicle which sounded like semi-automatic firearm inside the club and outside the south Orlando venue.

“We were just leaving the club and we started hearing the shots,” clubber Anthony Torres told the Daily News. “Everyone was running and screaming.”

Paramedic Jordan Hubler said, “There was a shooter with an assault rifle who shot up the people in the club sometime around 2 am. The shooter has himself barricaded inside Pulse with multiple hostages.”

The shooting came just over a day after a gunman fatally shot singer Christina Grimmie after a concert at the Plaza Live Theater in Orlando.

By Press TV