Islamic State have claimed responsibility for the Orlando nightclub shooting which killed 50 people and wounded 53 more, according to the Amaq news agency.
Meanwhile, US president Barack Obama has called the Orlando mass shooting at a gay nightclub an "act of terror" and an "act of hate".
The suspect, named as Omar Mateen from Port St Lucie, Florida, was armed with an assault-type rifle and handgun when he sprayed revellers with bullets at the popular venue, Pulse.
It is thought there were more than 300 people were inside the building at the time of the attack, which has now become the worst mass shooting in American history.
The president said, as Americans, they are "united in grief, in outrage and resolve to defend our people".
"The FBI is appropriately investigating this as an act of terrorism, and I directed that we must spare no effort to determine, what, if any, inspiration or association this killer may have had with terrorist groups," he added.
Mr Obama praised the emergency service response, described the gunman as a person "filled with hatred" and said it was a "heartbreaking day" for fellow Americans who are lesbians, gay, bisexual or transgender.
"This is a sobering reminder that attacks on any American regardless or race, ethnicity, religion or sexual orientation is an attack on all of us and the fundamental values and dignity that define us as a country," he added.
"This massacre is therefore a further reminder of how easy it is for someone to get their hands on a weapon that lets them shoot people in a school, in a house of worship or in a movie theatre or in a nightclub.
"We have to decided if that is the type of country we want to be. To actively to do nothing is a decision as well."
More than 53 wounded people have been taken to hospital following the incident at around 2am local time (7am BST), some of whom are thought to be critically injured.
The killer, who also held hostages in a three-hour stand-off, later died in a gunfight with Swat officers in the Florida city.
Officers in Orlando have since said their priority is now to begin the task of identifying victims.
The Florida government and and city of Orlando have both declared a state of emergency.
Denouncing the attack, Prime Minister David Cameron said: "I'm horrified by reports of the overnight shooting in Orlando. My thoughts are with the victims and their families."
Pope Francis expressed "horror" over the "homicidal folly" of the Florida nightclub massacre, and French president Francois Hollande said he "expresses the full support of France and the French with America's authorities and its people in this difficult time".
Florida governor Rick Scott called the shooting "heart wrenching", and said people who went into the building knowing there was an active shooter are heroic.