The Pentagon on Tuesday said NATO was "far from obsolete," defending the transatlantic alliance from Donald Trump Donald John TrumpObama calls on Senate not to fill Ginsburg's vacancy until after election Planned Parenthood: 'The fate of our rights' depends on Ginsburg replacement Progressive group to spend M in ad campaign on Supreme Court vacancy MORE's criticisms.

"NATO is far from obsolete. We think NATO is as relevant as ever right now in the current environment," said Pentagon press secretary Peter Cook.

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Trump, the GOP presidential front-runner, has criticized the 67-year old North Atlantic Treaty Organization" as unnecessary and expensive, arguing that the U.S. pays a disproportionate share of its cost.

"I think NATO is obsolete," Trump told ABC News on Saturday. "NATO was done at a time you had the Soviet Union, which was obviously larger -- much larger than Russia is today."

Trump said NATO should be "readjusted to take care of terrorism."

His remarks came after the March 22 terrorist attacks in Brussels, where at least 35 were killed and more than 300 injured. The Islamic State in Iraq and Syria has claimed responsibility.

Trump first called into question NATO's value during a interview with The Washington Post's editorial board a day before the attacks, saying NATO "as a concept is good, but it is not as good as it was when it first evolved."

“I’m not even knocking it, I’m just saying I don’t think it’s fair, we’re not treated fair.”

Rival GOP candidate Sen. Ted Cruz Rafael (Ted) Edward CruzSenate Republicans face tough decision on replacing Ginsburg Cruz: Trump should nominate a Supreme Court justice next week Renewed focus on Trump's Supreme Court list after Ginsburg's death MORE (R-Texas) has also criticized Trump for those remarks, saying that "abandoning Europe, withdrawing from the most successful military alliance of modern times makes no sense at all."

He added that withdrawing from NATO would "hand a massive victory to Putin" and a "massive victory" to ISIS.

"ISIS would be dancing in the street at the weakness and isolationism of Donald Trump," Cruz said.

Democratic presidential front-runner Hillary Clinton Hillary Diane Rodham ClintonWhat Senate Republicans have said about election-year Supreme Court vacancies Bipartisan praise pours in after Ginsburg's death Trump carries on with rally, unaware of Ginsburg's death MORE has also blasted Trump's remarks about NATO, saying that Russian President Vladimir Putin's hopes to divide Europe, and "If Mr. Trump gets his way, it will be like Christmas in the Kremlin."

Cook said, however, that it was important for NATO members to meet a target of spending two percent of their gross domestic product on defense.

"It's an important metric and one that we look to. The United States contributes a significant amount to -- to NATO and we look for other -- our allies to continue to do their part. But we think -- but we think, again, NATO's as relevant today as it's been in sometime," he added.

Cook touted Estonia, whose defense minister was scheduled to meet with Defense Secretary Ash Carter later in the day, as an example of a NATO partner living up to its commitment.

"Estonia has worked hard to live up to its commitment of NATO allies made in Wales to spend at least two percent of GDP on defense and Estonia is an important example of our NATO allies in both the areas of defense spending and host nation support," Cook said.

Cook said the United Kingdom has also stepped up its spending levels in recent months.

President Obama is planning to meet with NATO Secretary General Jens Stoltenberg at the White House on April 4 to discuss NATO's involvement in battling terrorism and the refugee crisis in Europe.

Cook said Carter would also meet with the secretary general as well, to discuss the attacks in Brussels, as well as fighting terrorism.

Several NATO allies are participating in the U.S.-led military coalition against ISIS, including Belgium, Canada, Denmark, France, the Netherlands, the United Kingdom, and Turkey.