Orlando/Washington: A gunman killed 50 people and injured 53 in a massacre at a gay nightclub in the tourist hub of Orlando, Florida, early on Sunday, the city’s mayor and police said, in an attack US authorities are described as a “terrorism incident."

The shooter was identified as Omar S. Mateen, a man that a senior Federal Bureau of Investigation (FBI) official said might have had leanings towards Islamic State militants. Officials cautioned that the Islamist tie required further investigation.

The death toll given by Orlando mayor Buddy Dyer to reporters made the attack the deadliest mass shooting in US history, eclipsing the 2007 massacre at Virginia Tech university, which left 32 dead.

A police officer working as a security guard inside the Pulse nightclub, which has operated in downtown Orlando since 2004, exchanged fire with the suspect at about 2 am (0600 GMT), police officials said.

A hostage situation quickly developed, and three hours later a squad of officers stormed the club and shot dead the gunman. It was unclear when the gunman shot the victims.

“Do we consider this an act of terrorism? Absolutely, we are investigating this from all parties’ perspective as an act of terrorism," said Danny Banks, special agent in charge of the Florida department of law enforcement. “Whether that is domestic terrorist activity or an international one, that is something we will certainly get to the bottom of."

US vice-president Joe Biden was briefed on the Orlando nightclub shootings by his national security adviser on Sunday and is closely monitoring the situation, the White House said in a statement.

“Biden offered his prayers for all those killed and injured in the shooting and sends his condolences to all the families and loved ones of the victims," the statement said. Reuters

Letitia Stein in Tampa, Colleen Jenkins in Winston-Salem, North Carolina, Chris Michaud in New York and Mary Milliken in Los Angeles contributed to this story.