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WASHINGTON — President Donald Trump called the murderous spree by a rental truck driver in New York City on Tuesday "another attack by a very sick and deranged person," sent condolences to victims of what the president referred to as a "terrorist attack," and pledged to step up extreme vetting of those seeking to enter the U.S.

Though ISIS has not claimed responsibility for the rampage, Trump warned in a tweet after being briefed by his chief of staff John Kelly: "We must not allow ISIS to return, or enter, our country after defeating them in the Middle East and elsewhere. Enough!"

A law enforcement official told WNBC that a note found in the suspect's truck claimed that he carried out the attack for ISIS.

In NYC, looks like another attack by a very sick and deranged person. Law enforcement is following this closely. NOT IN THE U.S.A.! — Donald J. Trump (@realDonaldTrump) October 31, 2017

We must not allow ISIS to return, or enter, our country after defeating them in the Middle East and elsewhere. Enough! — Donald J. Trump (@realDonaldTrump) October 31, 2017

The deadly incident is being considered by law enforcement officials to be a terror attack, which the president noted in later tweet and statement.

My thoughts, condolences and prayers to the victims and families of the New York City terrorist attack. God and your country are with you! — Donald J. Trump (@realDonaldTrump) October 31, 2017

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In statement from the White House, the president said, "Our thoughts and prayers are with the victims of today's terrorist attack in New York City and their families."

Later, Trump tweeted, "I have just ordered Homeland Security to step up our already Extreme Vetting Program. Being politically correct is fine, but not for this!"

The suspect was identified as a 29-year-old immigrant from Uzbekistan named Sayfullo Saipov, who entered the U.S. in 2010, law enforcement officials told NBC News.

Officials said he shouted "Allahu Akbar!"

White House Press Secretary Sarah Huckabee Sanders told reporters in an earlier statement that the president "will be continually updated as more details are known. Our thoughts and prayers are with all those affected."

Both first lady Melania Trump, who was in New York City at the time, and the president's daughter Ivanka, a White House senior adviser, tweeted about the attack, sending prayers to those affected.

My heart breaks for #NYC today. Thoughts & prayers as we monitor the situation. — Melania Trump (@FLOTUS) October 31, 2017

Heartbreaking news coming out of NYC. Praying for the victims. — Ivanka Trump (@IvankaTrump) October 31, 2017

Meanwhile, lawmakers condemned the apparent act of terror.

"This evening, all Americans were horrified and heartbroken by the despicable act of terror in Manhattan," House Minority leader Nancy Pelosi, D-Calif., said in a statement.

"As we await further news, we must continue to be smart and strong to keep the American people safe, not reckless and rash. Our nation's courage and resilience will always defeat the hatred and violence of terrorists," Pelosi added.

Speaker Paul Ryan, R-Wis., gave his thanks to first responders and offered prayers for those impacted.

Our hearts are with the victims of this apparent act of terror in New York City. Thank you to all the first responders. — Paul Ryan (@SpeakerRyan) October 31, 2017

Sen. Chuck Schumer, D-N.Y., said of the attack that took place just blocks from Ground Zero, "The scourge of terrorism is unfortunately still with us, and we must remain vigilant as ever."

Police said the driver of a rental truck deliberately drove onto a bike path on the west side of lower Manhattan hitting bicyclists and others before getting out of the vehicle and being shot by police, law enforcement officials and witnesses said.

At least eight people were killed and 11 injured. Saipov is hospitalized and in police custody.

Bill Clinton tweeted that New Yorkers "stand against terror."