DONALD Trump has called on US president Barack Obama to stand down over the Orlando shooting massacre, saying the US could not afford a 'politically correct president'.

Mr Trump said he was "right on radical Islamic terrorism", calling for toughness and vigilance.

The FBI has confirmed it twice investigated the Orlando gunman for his links to Islamic State but warned against making any definitive ink.

Omar Mateen, 29 year, who had firearms training for his job as a guard for a private security firm, was investigated by the FBI several years ago.

But he had not been under investigation or surveillance by the FBI at the time of the shooting.

The FBI said investigators first became aware of Mr. Mateen in 2013 "when he made inflammatory remarks to co-workers alleging possible terrorist ties.''

Mr. Hopper said the FBI "thoroughly investigated the matter, including interviews of witnesses, physical surveillance, and records checks."

Agents interviewed him twice, but "ultimately, we were unable to verify the substance of his comments and the investigation was closed.''

The following year, Mateen was investigated for  possible contacts with Moner Mohammad Abusalha, a Florida man who travelled to Syria and carried out a suicide car bombing attack.

Agents  found "that contact was minimal and did not constitute a substantive relationship or a threat at that time".

Mr Trump took to Twitter after the attack in an Orlando, Florida nightclub which killed 50 people and injured 53 others.

"Appreciate the congrats for being right on radical Islamic terrorism, I don't want congrats, I want toughness & vigilance," Mr Trump tweeted.

"We must be smart.

"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 in response to the massacre Mr Trump called on Mr Obama to step down.

"Because our leaders are weak, I said this was going to happen - and it is only going to get worse," Mr Trump said.

"I am trying to save lives and prevent the next terrorist attack. We can't afford to be politically correct anymore."

The FBI said Mateen might have leanings toward Islamic State's ideology but cautioned that the suspected Islamist connection required further investigation.

Mr Trump has proposed a temporary ban on Muslim immigration to the United States as a way to combat the threat of radical Islamist attacks.

Democratic nominee Hillary Clinton tweeted a brief statement after the attacks, but did not speculate on the motives of the gunman.

"Woke up to hear the devastating news from [Florida]. As we wait for more information, my thoughts are with those affected by this horrific act," Mrs Clinton wrote.