At least 50 people were killed and 53 others were injured in the shooting in the deadliest mass shooting in US history. A surgeon at Orlando Regional Medical Center said the death toll was likely to climb.
An FBI spokesman said the mass shooting is being investigated as an act of terrorism, adding that they are looking into whether this was an act of domestic or international terror, and if the shooter was a lone wolf.
Schiff said in a statement: 'This attack is so painfully reminiscent of the terrible attack at the Bataclan Theatre in Paris, and other ISIS-inspired attacks in recent years.
'The fact that this shooting took place during Ramadan and that ISIS leadership in Raqqa has been urging attacks during this time, that the target was an LGBT night club during Pride, and - if accurate - that according to local law enforcement the shooter declared his allegiance to ISIS, indicates an ISIS-inspired act of terrorism.
'Whether this attack was also ISIS-directed, remains to be determined.'
The gunman who slaughtered at least 50 people after taking party-goers hostage inside a gay club in Orlando had pledged allegiance to Islamic State, according to California congressman Adam Schiff.
Schiff, who serves as top Democrat on the House Select Committee on Intelligence, told CNN: 'What I've heard from the Department of Homeland Security this morning is that, according to local police, he made a pledge of allegiance to ISIL (ISIS).'
He said the shooting was 'highly indicative of an ISIL-inspired attack'.
Schiff added: '[He] was heard praying in a foreign language and I don't know if this was at some point during the course of the shooting but that's what I'm hearing, obviously second hand, coming ultimately from local police.'
Law enforcement sources have identified the shooter, who was wielding an AR-15 assault rifle and a handgun, as US citizen Omar Mateen, 29, from Port St. Lucie in Florida.
The gunman, who was born to Afghan parents, opened fire on the dance floor inside Pulse night club in the early hours of this morning.
But Mateen's father, Mir Seddique, told NBC News his son became angry when he saw two men kissing in Miami several months ago.
'This has nothing to do with religion,' he said. 'We are saying we are apologizing for the whole incident. We weren't aware of any action he is taking. We are in shock like the whole country.'
Writing on Facebook, Pulse, which described itself as the most prominent gay club in Orlando, urged party-goers to 'get out and keep running' as bullets started flying at around 2am local time.
Orlando Police Chief John Mina said the suspect exchanged gunfire with an officer working at the club around 2am, then went back inside and took hostages. There were about 320 people inside the club at the time of the shootings and about 100 people were taken hostage.
At around 5am authorities sent in a SWAT team to rescue the hostages. Nine hero officers used a 'controlled explosion' to distract the shooter before fatally shooting him and were able to rescue about 30 hostages who were hiding in the bathroom of the club.