"I think NATO as a concept is good, but it is not as good as it was when it first evolved. And I think we bear the, you know, not only financially, we bear the biggest brunt of it," he said. "I'm just saying I don't think it's fair. We're not treated fair."

Cruz seized on the comments Tuesday. Just hours after bombs went off at Brussels Airport and in a busy subway, Cruz said a Trump presidency would only weaken America.

"It is striking that the day after Donald Trump called for America weakening NATO, withdrawing from NATO, we see Brussels - where NATO is headquartered - the subject of a radical Islamic terrorist attack," Cruz said at a news conference in Washington. "Donald Trump is wrong that America should withdraw from the world and abandon our allies. Donald Trump is wrong that American should retreat from Europe, retreat from NATO, hand Putin a major victory and, while he's at it, hand ISIS a major victory."

Cruz also criticized President Barack Obama for what he called "political correctness."

"In the wake of Brussels, we don't need another lecture from President Obama on Islamophobia, we need a commander-in-chief who does everything necessary to defeat the enemy," he said.

"Today's attacks in Brussels underscore that this is a war. This is not an isolated incident, this is not a lone wolf, this is a war with radical Islamic terrorism. ISIS has declared jihad on Europe and on the United States of America."

At least 30 people died in Brussels after terrorists struck Tuesday, setting off two bombs at Brussels Airport and a third in a crowded subway. The country was on its highest terror alert on Tuesday.

ISIS has claimed responsibility for the attacks.