Friends and family members embrace outside the Orlando Police Headquarters during the investigation of a shooting at the Pulse nightclub, where people were killed by a gunman, in Orlando, Florida,June 12, 2016. [Photo/Agencies]

ORLANDO, Fla. - A gunman armed with an assault rifle killed 50 people at a packed gay nightclub in Orlando, Florida on Sunday in the worst mass shooting in US history which authorities described as a possible act of terrorism.

Police killed the shooter, who was identified as Omar S. Mateen, a 29-year-old Florida resident and US citizen. A top US congressman said Mateen may have pledged allegiance to the Islamic State militant group.

US officials cautioned, however, that they had no immediate evidence of any direct connection with Islamic State or any other foreign extremist group, nor had they uncovered any contacts between the gunman and any such group.

Fifty-three people were wounded in the rampage. It was the deadliest single US mass shooting incident, eclipsing the 2007 massacre of 32 people at Virginia Tech university.

"Today we're dealing with something that we never imagined and is unimaginable," Orlando Mayor Buddy Dyer said, more than doubling an earlier estimate that about 20 bodies had been found.

A police officer working as a security guard inside the Pulse nightclub exchanged fire with the suspect at about 2 a.m. (0600 GMT), authorities said. Pulse was crowded with some 350 revelers at a Latin music night.

"Everyone get out of pulse and keep running," the club's management wrote on Facebook as the incident unfolded.