WASHINGTON: Republican presidential candidate Donald Trump gave an aggressive response to the deadliest mass shooting in US history, quickly claiming the attack was the work of an Islamist militant while calling on President Barack Obama to resign and for Democrat Hillary Clinton to exit the presidential race.

For Trump, it was an attempt to frame the attack in Orlando in a light favourable to his campaign for the Nov 8 presidential election.

Early on Sunday, when few facts were known about the shooting, he boasted on Twitter that it proved he had been right about his warnings over “radical Islamic terrorism”. Trump cancelled a planned Monday evening rally in Portsmouth, New Hampshire late on Sunday, because of the shooting, but will go ahead with a major speech at St. Anselm's College on Monday afternoon.

In a tweet just hours after the incident, he wrote: “Appreciate the congrats for being right on radical Islamic terrorism, I don't want congrats, I want toughness and vigilance.”

Trump has made combating the threat of groups such as the militant Islamic State (IS) group a central part of his candidacy.

It was last December's attack in San Bernardino, California that killed 14 people that led Trump to propose a temporary ban on Muslims entering the United States. Trump re-visited the proposal on Sunday after at least 50 people died in the attack on a gay nightclub in Orlando, Florida.

“What has happened in Orlando is just the beginning. Our leadership is weak and ineffective. I called it and asked for the ban. Must be tough,” Trump wrote on Twitter.

In a statement late Sunday, the businessman went further than US officials investigating the shootings by asserting that the attack in Orlando was the work of a “radical Islamic terrorist”. The IS, which controls land in Iraq and Syria, claimed responsibility for the shooting but US officials said they had seen no immediate evidence directly linking the militant group to the massacre.

The shooter, US-born Florida resident Omar Mateen, was killed by police. Mateen, 29, called 911 on Sunday morning and made comments saying he supported the IS, officials said.

A Trump supporter played Trump's speeches on a loudspeaker outside the Port St. Lucie, Florida home of Mateen's parent's.

“I'm a Cuban citizen and I'm becoming an American citizen on July 6 and I'm going to vote for the first time in my life for Donald Trump,” said Arisley Travieso, 36, who works for a shredding company.

Traveiso said he supported Trump because the Republican nominee sees “the problem is Islamic terrorism. He (Trump) has been saying it nonstop”. Trump ripped Obama for his long-standing refusal to refer to IS and other extremist groups as in part representative of the religion of Islam.

“In his remarks today, President Obama disgracefully refused to even say the words 'Radical Islam'. For that reason alone, he should step down,” Trump said, adding that, “if Hillary Clinton, after this attack, still cannot say the two words 'Radical Islam' she should get out of this race for the presidency.”

Trump's day-long barrage of tweets and statements contrasted with more cautious approaches taken by Clinton, the presumptive Democratic nominee, and her fellow Democrat Obama.

In remarks at the White House, Obama said the investigation into the shootings was ongoing and declined to speculate on the motives of the shooter.

“We've reached no definitive judgment on the precise motivations of the killer,” Obama said. “The FBI is appropriately investigating this as an act of terrorism."

Similarly, Clinton, in a statement, called the attack an “act of terror” but did not speculate about the ideology of the gunman.

“Law enforcement and intelligence agencies are hard at work, and we will learn more in the hours and days ahead,” said Clinton, who expressed her sympathy for the victims and said “weapons of war” have no place on US streets.

Trump's campaign said that a speech scheduled for Monday that was to be devoted to criticising Hillary and Bill Clinton would now be centred on national security and counter-terrorism.