Republican Donald Trump is to further address the deadliest shooting in modern US history in a campaign speech which was originally intended to attack presumptive Democratic nominee Hillary Clinton.
The change of plan comes a day after Mr Trump called for Mrs Clinton to drop out of the race for president if she did not use the words "radical Islam" to describe the Florida nightclub massacre.
Mr Trump had planned to deliver remarks on Monday outlining his case against Mrs Clinton's potential presidency, but will instead alter his focus to "further address this terrorist attack, immigration and national security", his campaign said.
A gunman wielding an assault-style rifle and handgun opened fire inside a crowded gay nightclub in Orlando early on Sunday, killing at least 50 people before dying in a gunfight with police. Another 53 people were taken to hospital, most of them in a critical condition. Authorities identified the killer as Omar Mateen, a 29-year-old US citizen from Fort Pierce, Florida.
Mr Trump's hardline approach to fighting Islamic terrorism was a hallmark of his primary campaign.
The presumptive GOP nominee has proposed temporarily barring foreign Muslims from entering the country and advocated using waterboarding and other harsh interrogation methods to try to stave off future attacks.
In the hours after the Orlando shooting, he issued a statement calling on President Barack Obama to resign for refusing "to even say the words 'radical Islam'" in his response to the attack. He said Mrs Clinton should exit the presidential race if she does the same.
In an address from the White House, Mr Obama called the tragedy an act of terror and hate. He did not talk about religious extremists. He said the FBI would investigate the shootings in the gay nightclub as terrorism, but added that the gunman's motive was unclear.
Like Mr Obama, Mrs Clinton called the shootings acts of terror and hate, but did not use the words radical Islam in a statement released by her campaign. Instead, she said the country must "redouble our efforts" to defend itself, including "defeating international terror groups, working with allies and partners to go after them wherever they are, countering their attempts to recruit people here and everywhere, and hardening our defences at home".
Hours after Mr Obama spoke, a law enforcement official confirmed to The Associated Press that Mateen made a 911 call from the club during which he professed allegiance to the leader of the Islamic State group, Abu Bakr al-Baghdadi.
While some Republican leaders have encouraged Mr Trump to abandon his proposed Muslim ban in an effort to broaden his support among voters before November's general election, the Orlando attack appeared to harden the billionaire businessman's position.
"What has happened in Orlando is just the beginning," he tweeted on Sunday. "Our leadership is weak and ineffective. I called it and asked for the ban. Must be tough."
It was one of several tweets that drew criticism online for their told-you-so tone.
"Appreciate the congrats for being right on radical Islamic terrorism, I don't want congrats, I want toughness & vigilance. We must be smart!" he had written earlier.
Mrs Clinton's statement and tweets on Sunday made no mention of Mr Trump. She did make the case for gun control and reached out to a key constituency - gays and lesbians.
"To the LGBT community: please know that you have millions of allies across our country. I am one of them," she said.
On Friday, Mr Trump accused Mrs Clinton of failing to understand the gravity of the risk posed by Syrian refugees in a speech to a conservative Christian group.
"Hillary will bring hundreds of thousands of refugees, many of whom have hostile beliefs about people of different faiths and values and some of whom absolutely and openly support terrorism in our country," he said. "We don't need that ... We have enough problems right now."
Mrs Clinton has said she would support allowing more refugees from the war-torn Middle East to come to the US, as long as they are carefully screened and vetted.
Mr Trump will deliver his speech at Saint Anselm College in Manchester, New Hampshire, a possible battleground state, which gave him his first primary win. He cancelled a rally planned for Monday night in nearby Portsmouth.
Mrs Clinton's campaign postponed what would have been her first joint event with Mr Obama in Green Bay, Wisconsin, on Wednesday because of the shooting. She will campaign in Ohio on Monday.