US President Barack Obama on Sunday described a shooting at the Pulse nightclub in Orlando, Florida as the "most deadly shooting in American history."

Omar Mateen, a US citizen with Afghan roots, opened fire in a gay nightclub early Sunday morning, leaving at least 50 people dead and 53 others injured.

"Although it's still early in the investigation, we know enough to say that this was an act of terror and an act of hate," Obama said.

"This is a sobering reminder that attacks on any American, regardless of race, ethnicity, religion or sexual orientation, is an attack on all of us," the president added.

A law enforcement official told the AP news agency that the gunman had made a 911 call from the club, claiming allegiance to the leader of the so-called "Islamic State" Abu Bakr al-Baghdadi.

'We mourn the victims'

World leaders and government officials responded with outpourings of grief and comfort at the tragedy.

German Foreign Minister Frank-Walter Steinmeier expressed his condolences to the victims of the attack. "I am shocked by the murderous attack in Orlando. We mourn the victims. Our thoughts are with our friends in the USA," Steinmeier said in a tweet.

Israeli Prime Minister Benjamin Netanyahu said Israel "stands shoulder to shoulder" with the American people "in this tragic hour."

"In the name of the government of Israel and the citizens of Israel, I send our sincere condolences to the American people following the criminal attack on the LGBT (lesbian, gay, bisexual, transgender) community last night in Orlando," Netanyahu said in a statement issued in Hebrew.

Pope Francis, the head of the Catholic Church, expressed "the deepest feelings of horror and condemnation," while the UK's Queen Elizabeth said she and Prince Philip "have been shocked by the events in Orlando."

Solidarity

Meanwhile, former US State Secretary Hillary Clinton expressed her condolences, saying that the attack was not solely an act of terror, but also a hate crime.

"The gunman attacked an LGBT nightclub during Pride Month. To the LGBT community: please known that you have millions of allies across our country. I am one of them," Clinton said.

"We will keep fighting for your right to live freely, openly and without fear. Hate has absolutely no place in America," the presumptive Democratic presidential candidate added.

Presumptive Republican presidential candidate Donald Trump described the tragedy as a "horrific event."

ls/jm (AFP, AP, Reuters)