Donald Trump said the U.S. should rethink its involvement in NATO because the defense alliance costs too much money.

In remarks to CNN's Wolf Blitzer, Trump said the U.S. pays a disproportionate amount to NATO to ensure the security of allies.

"Frankly, they have to put up more money," he said. "We are paying disproportionately. It's too much, and frankly it's a different world than it was when we originally conceived of the idea."

For instance, Trump said Washington was "taking care" of Ukraine and that other European nations were not doing enough to support the Kiev government that has been locked in a long showdown with Moscow.

Donald Trump unveils foreign policy advisers

But Ukraine is not a member of NATO, and Washington is not providing arms to the government as it is fighting pro-Moscow rebels, though has provided nonlethal aid and has helped support international bailouts of the Ukrainian economy.

Later in the interview, Trump qualified his remarks saying that the U.S. should not "decrease its role" in NATO but should decrease its spending. Still, the Republican presidential front-runner's NATO comments could spur anxiety among the Western foreign policy establishment.

Hillary Clinton stepped up her attacks on Trump's character and behavior as she makes a case that he is not fit to be commander in chief -- an argument likely to form the centerpiece of the Democratic Party's assault if he becomes the Republican nominee.

"He has been engaging in bigotry and bluster and bullying, and I think when it comes to understanding what he would do as president, there are serious questions that have been raised, and this campaign should he be the nominee will have to address them," Clinton told CNN's Anderson Cooper.

The former secretary of state accused Trump of inciting violence and urging supporters to go after protesters in a way that she said "raises very serious questions." Asked whether Trump would behave differently as president, Clinton answered "Who knows?"

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