UPDATED 9.38AM A gunman has killed 50 people at a packed gay nightclub in Orlando, Florida in the worst mass shooting in US history.

Police killed the shooter, who was identified as Omar S. Mateen, a Florida resident.

The ex-wife of Omar Mateen has told media the 29-year-old was violent and mentally unstable and beat her repeatedly while they were married.

The woman, who spoke on condition of anonymity, says at first, the marriage was normal, but then he became abusive. She said he was not a stable person.

He owned a small-calibre handgun and worked as a guard at a nearby facility for juvenile delinquents.

Mateen called 911 on Sunday morning and said he supported the Islamic State militant group, officials said.

"It has been reported that Mateen made calls to 911 this morning in which he stated his allegiance to the leader of the Islamic State," said Ronald Hopper, the FBI's assistant special agent in charge on the case.

"We are looking into any and all connections, both domestic and international."

Islamic State's Amaq news agency says the Islamist militant group was responsible for the shooting.

"The armed attack that targeted a gay nightclub in the city of Orlando in the American state of Florida which left over 100 people dead or injured was carried out by an Islamic State fighter," Amaq said.

US officials cautioned, however, that they had no conclusive evidence of any direct connection with Islamic State or any other foreign extremist group.

Ronald Hopper said Mateen had twice before come to the attention of authorities. He said the first time was in 2013 when he made comments to his coworkers alleging terrorist ties.

"The FBI thoroughly investigated the matter, including interviews of witnesses, physical surveillance and records checks. In the course of the investigation, Mateen was interviewed twice. Ultimately we were unable to verify the substance of his comments, and the investigation was closed."

Mateen was investigated again in 2014 when the FBI looked into possible ties to a Florida man turned suicide bomber in Syria.

They concluded Mateen's contact with him was minimal, but Mateen remained on the FBI's watch list.

But Mateen's father said the shooting had nothing to do with religion and his family is in shock.

Seddique Mateen, also known as Mir Siddique, told media his son got angry when he saw two men kissing in Miami a couple of months ago and thinks that may be related to the shooting.

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

"We must spare no effort to determine what, if any, inspiration or association this killer may have had with terrorist groups," Obama said.

Fifty-three people were wounded in the rampage. It was the deadliest single US mass shooting incident in modern history, eclipsing the 2007 massacre of 32 people at Virginia Tech university.

Pulse was crowded with some 350 revellers at a Latin music night when the attack happened.

Clubgoer Joshua McGill described in a posting on Facebook how he fled the attack.

"I hid under a car and found one of the victims that was shot," McGill said, describing trying to bandage the victim with his shirt and quietly dragging him to a nearby police officer.

"Words cannot and will not describe the feeling of that. Being covered in blood. Trying to save a guy's life."

A hostage situation developed, and three hours later a team of SWAT officers used armoured cars to storm the club before shooting dead the gunman.

The number of dead shocked local officials, who had initially put the death toll at 20.

"Today we're dealing with something that we never imagined and is unimaginable," Orlando Mayor Buddy Dyer said. He said 39 people died inside the club, two outside, and nine others died after being rushed to hospital.

Mateen had twice been interviewed by FBI agents, in 2013 and 2014, after making comments to co-workers indicating he supported militant groups, but neither interview led to evidence of criminal activity, the FBI's Hopper said.

Likely Republican presidential nominee Donald Trump, who has called for a temporary ban on Muslims entering the US, said he was "right on radical Islamic terrorism."

He called in a tweet on Sunday for "toughness and vigilance."

Democratic candidate Hillary Clinton tweeted a brief statement after the attacks, but did not speculate on the motives of the gunman.

If confirmed as an act of terrorism, it would be the deadliest such attack on US soil since September 11, 2001, when al Qaeda-trained hijackers crashed jetliners into New York's World Trade Centre, the Pentagon and a field in Pennsylvania, killing some 3000 people.

The Orlando attacker was carrying an AR-15 style assault rifle and a handgun, Orange County Sheriff Jerry Demings said.

He also had an unidentified "device", said Orlando Police Chief John Mina.

The choice of target was especially heart-wrenching for members of the US lesbian, gay, bisexual and transgender community, said LGBT advocacy group Equality Florida.

"Gay clubs hold a significant place in LGBTQ history. They were often the only safe gathering place and this horrific act strikes directly at our sense of safety," the group said in a statement.

NZ response

No New Zealanders are believed to have been caught up in the Orlando shooting.

An MFAT spokesperson confirmed the New Zealand Embassy in Washington was in contact with local authorities.

“There is no information at this stage to suggest any New Zealanders have been caught up in the shooting.

“New Zealanders in the US are advised to follow any instructions issued by the local authorities and monitor the media to stay informed of developments.”

Prime Minister John Key said he would be writing a letter of condolence to US President Barack Obama.