Pictured: US citizen Omar Mateen, 29, from Port St. Lucie in Florida, slaughtered at least 50 people inside a gay club in Orlando

ISIS has claimed responsibility for the the deadliest mass shooting in US history and says the gunman who slaughtered at least 50 innocent party-goers in an Orlando gay club was one of its fighters.

Shooter Omar Mateen, 29, from Port St. Lucie in Florida, opened fire at Pulse night club in the early hours of Sunday, killing at least 50 people and injuring 53 others.

This is the first time ISIS has claimed responsibility for an attack on US soil.

Shortly before the attack, Mateen, who was born in New York to Afghan parents, called 911 and pledged allegiance to ISIS leader Abu Bakr al-Baghdadi, law enforcement officials told NBC News.

And the terror group’s Amaq news agency said on Sunday the Islamist militant group was responsible for the shooting.

‘The armed attack that targeted a gay night club in the city of Orlando in the American state of Florida which left over 100 people dead or injured was carried out by an Islamic State fighter,’ Amaq said.

U.S. officials cautioned, however, that they had no immediate evidence of any direct connection with ISIS or any other foreign extremist group, nor had they uncovered any contacts between the gunman and any such group.

During the attack, Mateen referenced the brothers who carried out the 2013 Boston Marathon bombing, a Massachusetts State Police spokesman said on Sunday.

‘During a conference call with federal law enforcement officials a short time ago, Massachusetts State Police and other local law enforcement authorities learned that the Orlando nightclub gunman, during his rampage, pledged allegiance to ISIS and referenced the Tsarnaev brothers,’ state police spokesman David Procopio said in an e-mail.

The Tsarnaev brothers killed three people and injured more than 260 in the April 15, 2013, attack. Tamerlan Tsarnaev, 26, died four days later during a gun battle with police while Dzhokhar Tsaranev, now 22, has been sentenced to death for his role in the attack.

However Mateen’s father, Mir Seddique, told NBC News the attack was ‘nothing to do with religion’ and said his son became angry when he saw two men kissing in Miami several months ago.

‘We are saying we are apologizing for the whole incident,’ he said. ‘We weren’t aware of any action he is taking. We are in shock like the whole country.’

So far, only seven victims of the massacre have been officially identified by the city of Orlando on its website.

They are: Edward Sotomayor Jr., 34; Stanley Almodovar III, 23; Luis Omar Ocasio-Capo, 20; Juan Ramon Guerrero, 22; Eric Ivan Ortiz-Rivera, 36; Peter O. Gonzalez-Cruz, 22; and Luis S. Vielma, 22.