ORLANDO, Fla., June 12 - A man armed with an assault rifle killed 50 people during a gay pride celebration at a nightclub in Orlando, Florida, early on Sunday in the deadliest mass shooting in U.S. history, a rampage President Barack Obama denounced as an act of terror and hate.

Police killed the gunman, who was identified as Omar Mateen, 29, a New York-born Florida resident and U.S. citizen who was the son of immigrants from Afghanistan and had twice been questioned by FBI agents in recent years.

Law enforcement officials were probing evidence suggesting the attack may have been inspired by Islamic State militants, although they said there was no proof that Mateen had worked directly with the group.

"It has been reported that Mateen made calls to 911 this morning in which he stated his allegiance to the leader of the Islamic State," said Ronald Hopper, the FBI's assistant special agent in charge on the case.

Shots rang out at the crowded Pulse nightclub in the heart of a city that is one of the most popular U.S. tourist destinations as some 350 patrons were attending a Latin music event in conjunction with gay pride week celebrations. Clubgoers described scenes of terror and pandemonium, with one man who escaped saying he hid under a car and bandaged a wounded stranger with his shirt.

"Words cannot and will not describe the feeling of that," Joshua McGill said in a posting on Facebook. "Being covered in blood. Trying to save a guy's life."

Fifty-three people were wounded in the rampage. It ranked as the deadliest single U.S. mass shooting incident, eclipsing the massacre of 32 people at Virginia Tech University in 2007.

"We know enough to say this was an act of terror, an act of hate," Obama said in a speech from the White House. "As Americans, we are united in grief, in outrage and in resolve to defend our people."

An undated photo from a social media account of Omar Mateen, who Orlando Police have identified as the suspect in the mass shooting at a gay nighclub in Orlando, Florida, U.S., June 12, 2016. Photo: Reuters

U.S. officials cautioned, however, they had no conclusive evidence of any direct connection with any foreign extremist group.

"So far as we know at this time, his first direct contact was a pledge of bayat (loyalty) he made during the massacre," said a U.S. counterterrorism official. "This guy appears to have been pretty screwed up without any help from anybody."

Still, the shooting was nearly certain to reignite emotional debates over American gun laws and homeland security in the midst of a U.S. presidential race shaping up to be a vitriolic campaign between Democrat Hillary Clinton and Republican Donald Trump.

The attack came six months after a married couple in California - a U.S.-born son of Pakistani immigrants and a Pakistani-born woman he married in Saudi Arabia, fatally shot 14 people in San Bernardino in an attack inspired by Islamic State. That couple died in a shootout with police hours after their attack on a holiday party attended by the husband's co-workers.

'Unimaginable'

The Florida shooting evolved into a hostage situation, which a team of SWAT officers ended around dawn when they used armored cars to storm the club before killing the gunman. It was unclear when the victims were killed.

The number of dead shocked officials in Orlando, a city of 270,000 people and home to tourist attractions including the Disney World resort. They had initially put the death toll at 20.

"We're dealing with something that we never imagined and is unimaginable," Orlando Mayor Buddy Dyer said. He said 39 people died inside the club, two outside, and nine others died after being rushed to hospital.

Orlando Regional Medical Center hospital said it had admitted 44 victims, including nine who died, and had carried out 26 operations on victims.

The city of Orlando, which drew 62 million visitors in 2014, began releasing names of the victims on Sunday, with the first four identified as Edward Sotomayor Jr., Stanley Almodovar, Luis Omar Ocasio-Capo and Juan Ramon Guerrero.

Mateen had twice been interviewed by FBI agents, in 2013 and 2014, after making comments to co-workers indicating he supported militant groups, but neither interview led to evidence of criminal activity, the FBI's Hopper said.

Hopper said Mateen was questioned in 2014 about his contacts with Moner Mohammad Abu-Salha, a U.S. citizen who also had lived in Florida and became a suicide bomber in Syria that year.

Following Sunday's shooting, a dozen unmarked police cars assembled around a house in the Atlantic coast town of Port St. Lucie, about 130 miles (210 km) southeast of Orlando that appeared to be linked to the gunman.

A man lays flowers at a memorial outside The Stonewall Inn on Christopher Street, considered by some as the center of New York State's gay rights movement, following the shooting massacre at Orlando's Pulse nightclub, in the Manhattan borough of New York, U.S., June 12, 2016. Photo: Reuters

Candidates weigh in

Within hours, the presumptive presidential nominees of both major political parties weighed in with statements on the shooting.

Trump, who has called for a temporary ban on Muslims entering the United States, said he was "right on radical Islamic terrorism" and called on Obama to resign because he did not say the words "radical Islam" in his statement responding to the shooting.

Clinton echoed Obama's comments calling the attack both an act of terror and a hate crime, adding that the massacre "reminds us once more that weapons of war have no place on our streets."

World leaders including Pope Francis, Britain's Queen Elizabeth and the leaders of Canada and Afghanistan condemned the attack.

If confirmed as an act of terrorism, it would be the deadliest such attack on U.S. soil since Sept. 11, 2001, when al Qaeda-trained hijackers crashed jetliners into New York's World Trade Center, the Pentagon and a field in Pennsylvania, killing some 3,000 people.

Mateen also referenced the ethnic Chechen brothers who killed three people and injured more than 260 in a bombing attack at the Boston Marathon in 2013, according to law enforcement officials.

The Orlando attacker was carrying an AR-15-style assault rifle and a handgun, Orange County Sheriff Jerry Demings said. He also had an unidentified "device," said Orlando Police Chief John Mina.

The choice of target was especially heart-wrenching for members of the U.S. lesbian, gay, bisexual and transgender community, said LGBT advocacy group Equality Florida.

"Gay clubs hold a significant place in LGBTQ history. They were often the only safe gathering place and this horrific act strikes directly at our sense of safety," the group said in a statement. "We will await the details in tears of sadness and anger."