The Islamic State (ISIS) terrorist organisation and its supporters are reportedly celebrating and praising the man who launched the deadliest terrorist attack on American soil single-handedly, calling him "one of the soldiers of the caliphate in America.''

Omar Mateen, 29, a U.S.-born citizen with Afghan parents, murdered at least 49 people and injured dozens more in a gay nightclub in Orlando, Florida on Sunday, claiming that he did it for ISIS, CBN News reports.

In a statement broadcast over its Al Bayan radio station, ISIS dubbed Mateen as "brother'' and called the shooting spree which the latter initiated "a security raid in which he was able to enter into a gathering of Crusaders in a nightclub for followers of the people of Lot (a reference to homosexuals),'' the Wall Street Journal (WSJ) reports.

The statement was translated by the global counter-terrorism organisation SITE Intelligence Group, according to WSJ

Several ISIS-linked Twitter accounts have also praised Mateen's actions, according to the Daily Mail. "One account posted Mateen's picture with the caption: 'The man who carried out the Florida nightclub attack which killed 50 people and injured dozens.''

Another account, now deleted, reportedly "gloated over the deaths using the #Pulse hashtag. 'We swear to Allah that we will kill you everywhere O disbeliever.''

Rita Katz, SITE director claimed the ISIS militants have been "celebrating the shooting as the best Ramadan gift.''

President Barack Obama said there is no definitive judgment yet on the precise motivations of the killer but assured that the FBI is "appropriately investigating this as an act of terrorism.''

Some 150,000 members of the LGBT community were believed to be in Orlando to celebrate Gay Pride Month, one of the largest pride events in the world. The city was wrapping up its celebration when terror struck the Pulse nightclub early Sunday, according to CBN News.

The club was packed with 300 patrons on Saturday night. By Sunday, Mateen held clubgoers hostage for three hours until SWAT teams stormed the establishment, reports say.

At least 53 other people were hospitalised, most of them reportedly in critical condition.

Mateen's ex-wife, Sitora Yusifiy described her former husband as "abusive and bipolar.''

"In the beginning he was a normal being that cared about family, loved to joke, loved to have fun. But a few months after we were married I saw that he was bipolar and he would get mad out of nowhere."

While denouncing the act, Nihad Awad, executive director of the Council on American-Islamic Relations, said Mateen does not represent the Muslim community in anyway.

"This is a hate crime, plain and simple. We condemn it in the strongest possible terms. It violates our principles as Americans and as Muslims. Let me be clear—we have no tolerance for extremism of any kind."

Special services and vigils are reportedly held in churches around Orlando throughout the week "to comfort and support the gathered mourners.''