FBI investigators have not been able to produce any persuasive evidence that Orlando gunman Omar Mateen was gay or pursuing gay relationships, sources say.

The gunman who killed 49 people in a shooting at Orlando's Pulse nightclub was not directed by a foreign terror group but was radicalised domestically, according to the FBI.

Orlando gunman Omar Mateen apparently made a series of Facebook posts before and during his attack on a gay nightclub, raging against the "filthy ways of the West" and blaming the US for the deaths of "innocent women and children".

The FBI is investigating reports that Orlando gunman Omar Mateen was a regular at the gay nightclub where he killed 49 people in America's worst mass shooting.

The gunman in the Orlando nightclub attack that killed 49 people had "strong indications of radicalisation" and was likely inspired by foreign terrorist organisations, the head of the FBI has said.