Republican candidate for the presidential nomination, Donald Trump, warned that the nuclear deal with Iran would lead to a doomsday scenario but said it would be difficult to just “rip up.”

In an interview that aired Sunday on NBC's "Meet the Press," he said that the deal would greatly strengthen Iran: "They are going to be such a wealthy, such a powerful nation. They are going to have nuclear weapons. They are going to take over parts of the world that you wouldn't believe. And I think it's going to lead to nuclear holocaust," Trump said. "Israel is in big trouble. Obama has really let Israel down."

But he said that as president, he may have to abide by it. "It's very hard to say, 'We're ripping it up,'" Trump explained.

Had he negotiated the deal, Trump insisted, he wouldn't have allowed Iran to access any of the estimated $150 billion in frozen funds that have been unfrozen by the deal; he would have demanded they return US prisoners before sitting down at the negotiating table; and he would have "doubled up" the sanctions to gain leverage. He also criticized the 24 day waiting period for nuclear inspectors, saying it amounts to much longer than that and in "24 days plus numerous weeks, you can do anything."

But while admitting he'd likely be stuck with the deal, Trump said he would at least "police the contract."

"I'm really good at looking at a contract and finding things within a contract that even if they're bad," he said, in an interview with NBC's Chuck Todd on his private plane.

"I would police that contract so tough that they don't have a chance. As bad as the contract is, I will be so tough on that contract."

Against ISIS, Trump expressed a tough stance, proposing another ground invasion of Iraq to take the oil fields from the terrorist groups, the proceeds from which he said he'd give to wounded soldiers and the families of soldiers killed in battle.

When told the Obama administration's current strategy for combating ISIS includes a similar plan, Trump insisted: "If they are trying to do it, they picked it up from me. Because I started saying it three weeks ago."