Donald Trump is not ruling out the use of nuclear weapons in the U.S. fight against the Islamic State of Iraq and Syria (ISIS), according to an interview with the GOP front-runner that aired on Bloomberg TV Wednesday.

Asked if he would consider nuclear retaliation after this week's terror attacks in Brussels, Trump responded: "Well, I'm never gonna rule anything out. And I wouldn't wanna say. Even if I felt -- it wasn't going -- I wouldn't wanna tell you that because, at a minimum, I want them to think maybe that we would use it."

"The fact is that we need unpredictability," Trump said of his openness to using nuclear weapons. "When you ask a question like that, it's a very - it is a very sad thing to have to answer it because the enemy is watching and I have a very good chance of winning. I frankly don't want the enemy to know how I'm thinking. But with that being said, I don't rule out anything."

When pressed on whether he would have utilized America's nuclear capabilities after September 11, Trump said that he would only have used it as a "last resort."

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During his interview, Trump emphasized that terrorists "are winning" and "we don't do anything about it."

"We're worried about laws and we can't waterboard and yet, they are chopping off heads and drowning people in steel cages," he told Bloomberg, adding in concerns that Muslims in the Middle East "do not respect us."

Trump's comments come on the heels of other controversial foreign policy proposals in recent days.

Earlier this week, the GOP front-runner proposed rethinking U.S. involvement in the NATO international alliance -- remarks for which he received widespread Republican criticism.

On Tuesday, Trump told CNN, "Frankly, they have to put up more money....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."

Trump doubled down on his NATO remarks in the Bloomberg interview as well, saying that "I think that NATO may be obsolete....It doesn't really help us. It's helping other countries. And I don't think those other countries appreciate what we are doing."