The Islamic State group on Monday claimed responsibility for the Orlando shooting that killed 50 people, saying in a radio bulletin that it was carried out by "one of the soldiers of the caliphate".

"God allowed Omar Mateen, one of the soldiers of the caliphate in America, to carry out an attack entering a crusader gathering in a night club... in Orlando in Florida, killing and wounding more than 100 of them," a bulletin from Al-Bayan radio said.

The jihadist group declared its "caliphate" in Syria and Iraq in 2014.

The IS-linked news agency Amaq said on Sunday that the attack had been "carried out by an Islamic State fighter".

US media reported that the gunman -- identified as 29-year-old Omar Mateen, an American citizen of Afghan descent -- had pledged allegiance to the jihadist group IS.

The FBI has admitted that Mateen -- who was born to Afghan parents in New York -- had previously been investigated, but cleared, for ties to a US suicide bomber.

The worst mass shooting in US history left 50 people dead, in addition to the shooter, and wounded 53 others.

US President Barack Obama said the FBI was investigating the killings "as an act of terrorism".

The father of the suspect said his son may have been motivated by homophobia.

Congressman Michael McCaul, chairman of the House Homeland Security Committee, dubbed the murders "the worst terrorist attack on American soil since 9/11."

The attack coincides with gay pride month in the United States, with festive events all over the country, including in Orlando last week.