"In seven years, that deal will have expired and Iran is free to go ahead and create nuclear weapons," Trump told a joint press conference with his Nigerian counterpart Muhammadu Buhari.

"That is not acceptable," Trump charged. "This is an agreement that wasn't approved by too many people. And it's a horrible agreement for the United States."

The US president has been threatening for months to pull out of the 2015 deal negotiated between Tehran and six world powers, rejecting it as "insane" partly because its restrictions on Iran's nuclear activities begin expiring in 2025.

Most world powers say the deal is working as intended for now and is the best way to keep Iran from acquiring the bomb.

"I'm not telling you what I'm doing, but a lot of people think they know," Trump said. "On or before the 12th, we'll make a decision."

Israeli Prime Minister Benjamin Netanyahu -- a fierce critic of the accord who has called for it to be altered or scrapped -- asserted Monday he had new "proof" of an Iranian nuclear weapons plan that could be activated at any time.

Netanyahu leveled the accusations in a televised address, but did not provide evidence that Tehran had actively worked to obtain an atomic weapon since the 2015 deal was struck.

The US president remained evasive when asked about the new Israeli charges.

"What Israel has done today in the news conference was right," Trump said, without elaborating.

The US president also rebutted the suggestion that walking away from the Iran deal would send a disastrous signal to North Korea -- as Washington seeks to persuade Pyongyang to give up its nuclear weapons.

"No, I think it sends the right message," Trump said.