Fifty people have been killed in a shooting in a gay nightclub in Orlando, Florida, the city’s mayor Buddy Dyer has said.
Omar Mateen, who was named as the attacker by officials, was killed after taking hostages at the Pulse club.
President Barack Obama said Americans were united in grief, outrage and “resolve to defend our people” after “an act of terror and an act of hate”.
The Orlando attack is the worst mass shooting in recent US history.
A nationwide moment of silence will be held at 18:00 local time.
According to officials, the killings were likely to be ideologically motivated, though there was no information that the gunman was associated with a particular group.
“It may be we’ve seen the commission of an awful hate crime,” he added.
Omar Mateen’s father Mir Seddique told NBC News that the incident had nothing to do with religion, and may have been triggered by the sight of a gay couple kissing in Miami.
However, NBC News reported that Omar Mateen called the emergency services before the attack and swore allegiance to ISIS.
The group later said – via its affiliated Amaq news agency – that an ISIS “fighter” had carried out the attack.
The claim did not specify whether ISIS was directly involved or simply taking credit for inspiring the attack.
President Obama described the attack as “the brutal murder of dozens of innocent people”.
“It was an act of terror and an act of hate,” the president said, adding that it was a heartbreaking day for the LGBT community.
An attack on any American was an attack on all, Barack Obama added.
“We are united in grief, in outrage and resolve to defend our people.”
Barack Obama has ordered flags on federal buildings to be flown at half mast until sunset on June 16.
The suspect, who was a US citizen from the Florida town of Port St Lucie and was of Afghan descent, was not on a terrorism watch list.
However, officials revealed that the FBI had twice interviewed Omar Mateen in 2013-2014 after he made “inflammatory remarks” to a colleague. But the investigation was closed.
The death toll of 50 given by Mayor Buddy Dyer means that the Orlando attack surpasses the 2007 massacre at Virginia Tech which left 32 people dead.
Another 53 people were injured in the shooting.
Security has also been boosted for the last day of the Capital Pride LGBT festival in Washington DC, which included a moment of silence for the Orlando victims at 13:00 local time, the Washington Post reported.
A gay pride parade has also been taking place in West Hollywood, Los Angeles.