Washington: Omar Mateen, the Afghan-origin gunman who killed at least 50 people at a gay nightclub in Orlando in US' deadliest mass shooting incident, pledged allegiance to ISIS in a phone call to emergency number 911 during the attack.

Mateen, a 29-year-old security guard who was born in New York to Afghan parents, called 911 after his initial assault on the dance club to pledge allegiance to the leader of the Islamic State Abu Bakr al-Baghdadi, law enforcement officials were quoted as saying by media reports.