ISIS claims responsibility for Orlando nightclub shooting

ISIS's Amaq news agency said on Sunday that the Islamist terrorist group was responsible for the shooting that killed at least 50 people in a massacre at a gay nightclub in Orlando, Florida.

"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 ISIS fighter," Amaq said.

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

"We know enough to say this was an act of terror, an act of hate," Obama said in a speech from the White House. "And as Americans, we are united in grief, in outrage and in resolve to defend our people."

U.S. officials have reached no definitive judgment on the killer's precise motives, Obama added. "We must spare no effort to determine what, if any, inspiration or association this killer may have had with terrorist groups," he said.

It was the deadliest single U.S. mass shooting incident, eclipsing the 2007 massacre of 32 people at Virginia Tech university. Pulse was crowded with some 350 revelers at a Latin music night when the attack happened.

"Everyone get out of pulse and keep running," the club's management wrote on Facebook as the incident unfolded.

A hostage situation developed, and three hours later SWAT team officers used armored cars to storm the club before shooting dead the gunman. It was unclear when the victims were killed.

Video footage showed police officers and civilians carrying people away from the club and bending over others on the ground. Dozens of police cruisers, ambulances and other emergency vehicles could be seen in the area.

"Today we're dealing with something that we never imagined and is unimaginable," Orlando Mayor Buddy Dyer said, more than doubling an earlier estimate that about 20 bodies had been found.

Dyer said 39 people were killed inside the club, two outside, and nine others died after being rushed to hospital. Dozens of terrified patrons, some of whom had been hiding in restrooms, were rescued. One officer was injured when he was hit in his helmet while exchanging fire with the gunman, police said.

U.S. Representative Adam Schiff, the top Democrat on a congressional intelligence committee said that according to local law enforcement the shooter had declared his allegiance to Islamic State.

He noted that the shooting took place during the Muslim holy month of Ramadan, and that Islamic State leaders who control territory in Syria and Iraq have urged attacks during this time. All of this indicates an ISIS-inspired act of terrorism, he said in a statement, referring to Islamic State.