US Republican presidential frontrunner Donald Trump took a strong departure from his party's rejection of the Iran nuclear deal, saying he would not necessarily "rip up" the accord, in an interview aired Sunday.

"I would police that contract so tough that they don't have a chance," the bombastic real estate mogul said in an interview on NBC's "Meet the Press," adding "it's very hard to say we're ripping up" the deal.

Several of Trump's fellow Republican contenders have publicly opposed the deal, with some vowing to abolish it if elected.

The Republican-controlled US Congress is expected in September to vote against the deal, a landmark agreement that would roll back Iran's nuclear program in exchange for an easing of crippling economic sanctions.

But the Republicans are unlikely to have enough support to overturn a subsequent veto by President Barack Obama.

Trump made the comments even as he sounded a dismal note on the deal's eventual outcome and called US Secretary of State John Kerry, who worked to negotiate it, incompetent.

"Iran is going to be unbelievably powerful and unbelievably rich and Israel is in big trouble," Trump said.

"They are going to be such a wealthy, such a powerful nation, they're 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.

"The people that negotiated that deal, namely Kerry and his friends, are incompetent," he said.

The billionaire businessman added that as an entrepreneur, however, he was "good at looking at a contract and finding things in a contract even though they're bad."

"I've heard a lot of people say 'We're going to rip up the deal.' It's very tough to do when you say 'rip up a deal' because I'm a deal person," Trump said.

The money that could become available to Iran with the easing of sanctions could be as much as $150 billion, US media has reported.

Asked specifically whether he would keep the deal alive as president, Trump responded: "The problem is by the time I got in there, they will have already received the $150 billion."