US President Barack Obama has reacted to the terror attack at Pulse nightclub in Orlando, Florida, which left 50 people dead and a further 53 people in hospital.

Speaking in the White House on Sunday the President said “We pray for the families who are grasping for answers... We stand with the people of Orlando, who have endured a terrible attack on their city,”

“This is an especially heartbreaking day for all of our friends, our fellow Americans, who are lesbian, gay, transgender and bisexual.”

The President said the attack was against more than just a nightclub. He said Pulse was a place where “people came to dance and to sign and to live” and “a place of solidarity and empowerment, where people have come together to raise awareness to speak their minds and to advocate for their civil rights”.

He labelled the attack a “sobering reminder that attacks on any American, regardless of race, ethnicity, religion or sexual orientation, is an attack on all of us.”

The gunman, named by officials as Omar Mateen, was killed after taking hostages.

The attack was the worst mass shooting in recent US history.

Officials have said the killings were likely to be ideologically motivated, though there was no information that the gunman was associated with a particular group.

NBC news is reporting that Omar Mateen called 911 before the attack and swore his allegiance to the leader of terror group Isis.

Obama had a meeting with FBI director James Comey and other intelligence officials, and that he ordered “the full resources” of the government into the investigation. 

“We are still learning all the facts. This is an open investigation. We have reached no definitive judgment on the precise motivations,” he adds. “What is clear is that this was was a person filled with hate.”

President Michael D Higgins sent his condolences to the families of the victims of the shooting.

"Irish people will want to join with so many others who are horrified and deeply saddened to learn of the shootings in Orlando earlier today which have resulted in such loss of life and injury.

"As President of Ireland I wish to express my sincerest condolences to the families of all those bereaved and affected by today's tragedy.

"Our thoughts are with the people of Florida and the community in Orlando and Orange County at this difficult time. The loss of innocent life on such a horrendous scale is truly shocking and challenges us all.

"I have spoken to Ambassador O'Malley and personally conveyed my deepest sympathy and our nation's solidarity with the people of the United States at this time."

Cities around the US are rallying behind Orlando, offering assistance and stepping up their security around LGBT locations in their own precincts.

In Los Angeles, police have arrested a man with an assault rifle, other weapons and possible explosives – he said he was headed to the city’s pride parade, the Los Angeles Times reports.

New York Mayor Bill de Blasio ordered all US flags at half-staff as a mark of respect for the victims, and City Hall will be lit in rainbow colours “to represent LGBT pride at sunset”.