The wife of the gunman who killed 49 people at an Orlando gay nightclub could face criminal charges after a federal grand jury was convened to study possible wrongdoing by her, a law enforcement source says.

Vigils have been held in Orlando, Florida, and around the world for the victims of Sunday's deadly gun attack on a gay nightclub.

Prime Minister Sheikh Hasina strongly condemns and expresses deep shock at the death of 50 people in a terrorist attack on a nightclub in the USA.

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.