Mass shooting at Florida club treated as 'terrorist attack'

The exact death toll continued to rise in what appears to be the worst mass shooting in US history.

The gunman was identified by US television networks as Omar Mateen from Port St Lucie in Florida, born to Afghan parents in 1986.

The building attacked has been cleared and the wounded rushed to hospitals.

"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," Orlando Mayor Buddy Dyer told a news briefing, raising the death toll from 20 previously reported.

"There are another 53 that are hospitalised," Dyer added.

Officials were looking into whether the attack was an act of domestic or international terror, and if the shooter acted alone, according to Danny Banks, an agent with the Florida Department of Law Enforcement.

"This is an incident, as I see it, that we certainly classify as domestic terror incident," Orange County Sheriff Jerry Demings said.

An Islamic State [IS] group affiliated Twitter account posted pictures of the shooter identified as Mateen, Reuters reported.

But US media reported Mateen's father saying the attack had "nothing to do with religion," the agency added.

The incident comes just 21 days after the IS released a message calling for attacks on the West during the Islamic holy month of Ramadan, which started on Monday.

The militant group called on attacks against ordinary civilians living in the West, claiming in the 31-minutes audio clip that "there are no innocents in the heart of the lands of the Crusaders."

US authorities investigating the incident said they are "confident" that there are no additional threats to the country.

Agencies contributed to this report.