ORLANDO: Fifty people died in the worst mass shooting in US history on Sunday when a gunman opened fire at a packed gay nightclub in Orlando, Florida.

The FBI said they were investigating possible `radical Islamic leanings` of the suspect, identified by US media as 29-year-old Omar Mateen, a US citizen of Afghan descent.

President Barack Obama decried the shooting as a terrorist acttargetingaplace of `solidarity and empowerment` for gays and lesbians.

Hours after the massacre, the president said in an address to the nation that the FBI would investigate the shooting as terrorism, but added the shooter`s motivations were unclear.

The suspect`s father told NBC news that his son might have been motivated by homophobia and not by his faith.

`This had nothing to do with religion,` Mir Seddique told the network, recalling a recent incident in Miami.

`He saw two men kissing each other in front of his wife and kid and he got very angry,` he said, apologising to the victims.

A news agency linked to the militant Islamic State (IS) claimed that the attack was carried out by a member of the group.`The attacl< that targeted a nightclub for homosexuals in Orlando and that left more than 100 dead and wounded was carried out by an Islamic State fighter, the Amaq agency said in a terse statement quoting a `source`.

White House hopeful Donald Trump lost no time in claiming the massacre showed he has been right about `Islamist radicalism`.

In a series of tweets starting just hours after the shooting began, Trump said the tragedy supported his views on the threat facing America.

`Appreciate the congrats for being right on radical Islamic terrorism,` he said.

`I don`t want congrats, I want toughness and vigilance. We must be smart! Terrified survivors described how the gunman raked the club with bullets, prompting a police SWAT team to storm the venue and shoot him dead.

Officials said many of the victims were in critical condition, so the death toll could yet rise.

FBI official Ronald Hopper told reporters officials were `confident` there was no immediate further threat to the area, or to the US.

Omar Mateen was born to Afghan parents in 1986 and lived in Port St Lucie, Florida, about two hours` drive from Orlando.

CBS News reported that Mateen had no apparent criminal history.

`Worst attack since 9/11` Hopper, the FBI official, said authorities were looking into whether the suspecthad `leanings` towards Muslim extremists.

Events unfolded over a three-hour period from around 2am (llam in Pakistan) when shots rang out amid the throbbing music at the Pulse Orlando club.

Police said the suspect was armed with an assault rifle and a handgun. A police officer working `extra dutles` at the club responded, joined by two other officers, who exchanged fire with the suspect.

`The suspect at some point went back inside the club where more shots were fired. This did turn into a hostage situation,` police chief John Mina said.

`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 armoured vehicle known as a BearCat.

About 30 people were rescued during the operation, the police chief claimed.

It was the second shooting in the city in just two days.

Singer Christina Grimmie was shot dead on Friday by a gunman at a theatre after a gig.

Speaking to Sky News, clubber Ricardo Negron, who was inside when the shooting began, described how the gunman sprayed the club with bullets.

`People just dropped on the floor. I guess the shooter was shooting at the ceiling because you could see all the glass from the lamps falling,` he told the network.-Agencies