Orlando shooting: Donald Trump says on Twitter that he was right on 'Islamic terrorism'

US Republican presidential candidate Donald Trump, hours after the Orlando nightclub shooting, said on Sunday that he was right about "radical Islamic terrorism."

"Appreciate the congrats for being right on radical Islamic terrorism, I don't want congrats, I want toughness & vigilance. We must be smart!" the billionaire businessman said in a posting on Twitter.

Appreciate the congrats for being right on radical Islamic terrorism, I don't want congrats, I want toughness & vigilance. We must be smart!

— Donald J. Trump (@realDonaldTrump) June 12, 2016

A gunman killed 50 people at a packed gay nightclub in Orlando, Florida on Sunday and wounded 53 others before being killed by police in the worst mass shooting in US history.

The shooter was identified as Omar S. Mateen, a Florida resident who a senior FBI official said might have had leanings towards Islamic State militants. Officials called the rampage a "terrorism incident," but cautioned that the suspected Islamist connection required further investigation.

Orlando Mayor Buddy Dyer said the attack was the deadliest single US shooting incident, eclipsing the 32 people killed in the 2007 massacre at Virginia Tech university. "Today we're dealing with something that we never imagined and is unimaginable," Dyer said, more than doubling an earlier estimate that about 20 bodies were found in the Pulse nightclub.