Americans were united in "grief, outrage and resolve to defend our people", he said.

Omar Mateen, 29, killed 50 people and wounded 53 at the Pulse club before being shot dead by police.

The so-called Islamic State group has said it was behind the attack, but the extent of its involvement is not clear.

A statement on its affiliated Amaq news agency said that an IS "fighter" was responsible.

NBC News reported that Mateen had called the emergency services before the attack and swore allegiance to IS.

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

Gun control
Mr Obama said the "brutal murder of dozens of innocent people" was a further reminder of how easy it was to acquire a deadly weapon in the US and shoot people.

"We have to decide if that's the kind of country we want to be," said the president, who has campaigned for stricter gun laws. "To actively do nothing is a decision as well."

He added that it was a heartbreaking day for the LGBT community, and that an attack on any American was an attack on all.

Mr Obama has ordered flags on federal buildings to be flown at half mast until sunset on Thursday.

(BBC News)