Families of some of the 49 people killed in a massacre at an Orlando gay nightclub will mourn and bury their dead today, a day after President Barack Obama meets survivors and says the United States must act to control gun violence.

Wife of gunman who killed 49 people at an Orlando gay nightclub knew of his plans for the attack and could soon be charged in connection with the deadliest mass shooting in modern US history, a law enforcement source said on Tuesday.

The gunman who attacked a Florida LGBT nightclub had visited the club at least a dozen times before carrying out his attack, witnesses said.

White House hopeful Donald Trump lost no time Sunday in claiming a massacre in a gay nightclub showed he has been right about Islamic extremism.

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.