UPDATE: The shooter who killed at least 50 people in a LGBT nightclub in Florida called emergency services and swore allegiance to the head of ISIS, according to police.
Omar Mateen, who reportedly called 911 before the attack, shot down at least 50 people at the Pulse nightclub.
Islamic State, which is strongly anti-LGBT, has allegedly claimed responsibility for the shooting, according to ISIS-affiliated news agency Amaq, although an official claim from ISIS has been disputed.
President Barack Obama said the shooter was a man "filled with hate" and called the incident the worst shooting in US history.
The 29-year-old, who was born in New York, was equipped with a handgun and an assault rifle.
The Council on American-Islamic Relations, the largest advocacy group for Muslims in the US, has asked the Muslim community to donate blood to those injured in the attack.
Rasha Mubarak, the Florida chapter's Orlando regional co-ordinator, said: "We condemn this monstrous attack and offer our heartfelt condolences to the families and loved ones of all those killed or injured.
"The Muslim community joins our fellow Americans in repudiating anyone or any group that would claim to justify or excuse such an appalling act of violence."
Mateen was also accused of beating his ex-wife regularly before the attack.
Meanwhile the City of Orlando has begun paying tribute to those killed in the horror shooting, progressively releasing names of those who lost their lives.
On this very difficult day, we offer heartfelt condolences to today's victims and their families.
Our City is working tirelessly to get as much information out to the families so they can begin the grieving process. Please keep the following individuals in your thoughts and prayers. #PrayforOrlando
The below list of individuals includes victims who have lost their lives during the early morning incident, and next of kin have been contacted. As we continue to reach out to the families of victims, we will continue to update this post.
Edward Sotomayor Jr., 34 years old
Stanley Almodovar III, 23 years old
Luis Omar Ocasio-Capo, 20 years old
Juan Ramon Guerrero, 22 years old
Eric Ivan Ortiz-Rivera, 36 years old
Peter O. Gonzalez-Cruz, 22 years old
50 killed and 50 injured in America's deadliest single shooting
At least 50 people were killed and 50 injured when a gunman opened fire with an assault rifle in a LGBTI bar in Orlando.
The attack appears to be the worst mass shooting in American history outside of war.
The lone man, identified as Omar Mateen, went into the nightclub and started firing a weapon.
Within an hour police set off an explosion after apparent negotiations.
The gunman is now dead.
Police are investigating whether there is any tie between Mr Mateen and any jihadist organisations.
Police believe there is an "ideology connected" with the shooting.
No foreign organisation is taking credit, but terrorist group ISIS has released tweets gloating over the attack.
Mr Mateen was born in the United States and is a divorcee.
His father has released a statement saying it was not a jihadist attack but rather that the security guard was repulsed by homosexuality.
The attack is being characterised as both a hate crime and a terrorist attack.
CNN reports the gunman called 911 to pledge allegiance to ISIS and specifically mentioned the Boston Marathon bombing.
The shooting took place at the Pulse Nightclub, a high-energy warehouse dance club featuring DJs, LED lighting, VIP tables and a huge dance floor.
Statement from Press Secretary Josh Earnest:
The President was briefed this morning by Lisa Monaco, Assistant to the President for Homeland Security and Counterterrorism, on the tragic shooting in Orlando, Florida. Our thoughts and prayers are with the families and loved ones of the victims. The President asked to receive regular updates as the FBI, and other federal officials, work with the Orlando Police to gather more information, and directed that the federal government provide any assistance necessary to pursue the investigation and support the community.
Statement from Vice President Joe Biden's spokesperson:
The Vice President was briefed this morning by his national security advisor on the heinous attack that took place overnight at a nightclub in Orlando, Florida. Vice President Biden offered his prayers for all those killed and injured in the shooting and sends his condolences to all the families and loved ones of the victims. He is closely monitoring the situation and will continue to receive regular updates as we know more.
UPDATE: Authorities say up to 20 people died and 42 were hospitalised as a result of the mass shooting at an LGBTI bar in Orlando.
Police have also confirmed the gunman responsible for the incident which happened about 3am (local time) is now dead and the incident is being treated as a suspected act of terrorism.
EARLIER: POLICE are searching the area around a hospital in Orlando, Florida after a mass shooting at a gay nightclub about 3am (local time).
Witnesses who were inside the busy club have described a man walking in and firing shots into the ceiling and then the crowd.
As the incident began to unfold the Pulse Nightclub posted on its Facebook page saying: "Everyone get out of pulse and keep running."
Dozens of emergency vehicles surrounded the chaotic scene amidst unconfirmed reports of as many as 20 casualties.
One witness told Sky News more than 100 people were inside the venue attending a Latin night when the gunman entered the building and opened fire.
The witness could not confirm if there was one gunman or several.
"We just heard shots, it was less than a minute, it felt longer," Ricardo Negron said.
"There was a brief pause and we just ran. Everybody was just faces to the floor. I had someone over me, I really didn't see [how many gunmen there were].
"All I can tell you was the club was packed, there were over 100 people there."
The Guardian reports another witness as saying: "They are just pulling people out in stretchers loading them up," he said.
"Please god let everyone make it - tonight was supposed to be a fun night!! One more minute and we would of been shot or worse."
More to follow.
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