President Obama on Sunday called a mass shooting in a crowded gay nightclub in Orlando, Fla., “an act of terror and hate,” vowing a swift federal investigation into the attack.

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In a brief, somber statement from the White House, Obama said that the FBI is investigating the shooting as an act of terror. The gunman's motives, however, are not yet clear, he said.

"This was an act of terror and an act of hate,” Obama told reporters in the press briefing room. “And as Americans, we are unified in grief and outrage and in resolve to defend our people.”

Obama noted the shooting was carried out at a gay club during LGBT Pride Month.

“The place where they attacked is more than a nightclub. It is a place of solidarity and empowerment,” he said.

He said the incident is a “sobering reminder that attacks on any American, regardless of race, religion or sexual orientation, is an attack on all of us and on the fundamental values of equality and dignity that define us as a country.”

Fifty people were killed and 53 others were injured early Sunday morning when a gunman opened fire inside the Pulse nightclub in the deadliest mass shooting in U.S. history.

Federal law enforcement officials say the gunman was Omar Mateen, a 29-year-old American citizen living in Port St. Lucie, Fla. He was killed at the scene in a shootout with police.

He was reportedly armed with an AR-15-style assault rifle and a handgun.

News networks reported Mateen phoned 911 moments before the shooting to pledge allegiance to the Islamic State in Iraq and Syria (ISIS).

But it remains unclear if the planning and execution of the attack was directly linked to foreign terrorist groups.

The shooting forces Obama to confront two intractable issues that have dogged his presidency: mass gun violence and terrorism.

Sunday marked at least the 20th time the president made a statement on a shooting spree, according to CBS News reporter and unofficial White House historian Mark Knoller.

Last December, the president struggled to respond to terror attacks in Paris and San Bernardino, Calif., that were carried out by armed, radicalized assailants.

Obama has come under fire from Republicans — and some Democrats — for not doing enough to counter the threat posed by overseas terror networks such as ISIS, which seek to inspire individuals in the U.S. to carry out attacks.

That debate has intensified during the presidential election.

"Is President Obama going to finally mention the words radical Islamic terrorism? If he doesn't he should immediately resign in disgrace!" presumptive GOP presidential nominee Donald TrumpDonald John TrumpGrassroots America shows the people support Donald Trump Trump speaks to rebel Libyan general attacking Tripoli Dem lawmaker: Mueller report shows 'substantial body of evidence' on obstruction MORE tweeted during Obama's speech.

But Obama urged the public not to blame the Muslim community as a whole for the attack.

“In the face of hate and violence, we will love one another,” he said. “We will not give into fear or turn against each other. Instead, we will stand united as Americans to protect our people and defend our nation and to take action against those who threaten us.”

Obama renewed his call on Republicans in Congress to do more to keep guns out of the hands of suspected terrorists.

He said the Orlando shooting should serve as a “further reminder of how easy it is for someone to get their hands on a weapon that lets them shoot people in a school, or in a house of worship, or in a movie theatre, or in a nightclub.”

“We have to decide if that’s kind of country we want to be. And to do nothing is a decision as well.”

The president’s efforts to enact stronger federal gun laws in the wake of the 2012 shooting at an elementary school in Newtown, Conn., have been stymied in Congress.

Last December, the Senate blocked an amendment that would have allowed the administration to block the sale or transfer of firearms to people on terror watch lists.

Mateen had become known to the FBI in 2013 and 2014, according to the Daily Beast. But federal authorities reportedly closed an investigation into him after they discovered nothing that warranted a further probe.

The failure of gun-control legislation led the president to hand down a series of executive actions in January designed to bolster federal background check requirements and research on so-called smart-gun technology.

— Jessie Hellmann contributed to this report, which was updated at 2:30 p.m.