WATCH: Trump Declares 'Once Beautiful Brussels' Now a 'Disaster City'

Republican front-runner responds to Brussels terror attacks, 'This is just the beginning. It will get worse and worse because we are lax and foolish.'

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U.S. Republican presidential candidate Donald Trump, reacting to attacks at Brussels airport and a metro station on Tuesday, said the United States and Western countries should toughen up in their fight against Islamist militants.

"I would close up our borders," Trump told Fox News in an interview.

"We have to be very, very vigilant as to who we allowed into this country." -@realDonaldTrump

"We are lax and we are foolish," said Trump, the front-runner in the race for the Republican nomination in the November election

"I would close up our borders to people until we figure out what is going on." -@realDonaldtrump

In gave another interview to NBC's "Today" program, a day after he expressed skepticism about the U.S. role in the North Atlantic Treaty Organization and said the United States should significantly cut spending on the defense alliance.

"As president ... I would be very, very tough on the borders, and I would be not allowing certain people to come into this country without absolute perfect documentation," he told NBC.

Republican rival John Kasich, the governor of Ohio, struck a different tone, pledging to "redouble our efforts with our allies" and saying the United States "must strengthen our alliances" in the face of acts of terror.

U.S. Senator Ted Cruz, who is running second to Trump in the Republican delegate count, called the blasts in Belgium "the latest in a string of coordinated attacks by radical Islamic terrorists."

"Radical Islam is at war with us," he said in a statement posted online.

Adam Schiff, the top Democrat on the U.S. House Intelligence Committee, said the Brussels attacks that killed at least 26 people at the airport and a rush-hour metro train, "bear all the hallmarks of an ISIS-inspired, or ISIS-coordinated attack," using an acronym for Islamic State.

The attacks will likely revive national security as a key issue in the 2016 race for the White House, at least for now.

Trump looks to take another step toward winning the Republican presidential nomination in contests in Arizona and Utah on Tuesday, aiming to deal another setback to the party establishment's flagging stop-Trump movement.

The billionaire businessman has rolled up a big lead in convention delegates who will pick the Republican nominee, defying weeks of attacks from members of the party establishment worried he will lead the Republicans to defeat in the Nov. 8 presidential election.

In Arizona, one of the U.S. states that borders Mexico, Trump's hardline immigration message is popular and he leads in polls, while in Utah Trump lags in polls behind Cruz, a U.S. senator from Texas.