Nixing Iran deal would worsen North Korea situation: John Kerry

US President Donald Trump risks driving Iran towards nuclear proliferation and worsening a standoff with North Korea if Washington ends a nuclear deal with the Islamic republic, former Secretary of State John Kerry said on Thursday.

Kerry, who negotiated the 2015 nuclear deal between Iran and world powers, was speaking a week after Trump refused to certify that Tehran was in compliance, and amid growing tensions with Pyongyang over its nuclear and ballistic missile programmes.

"If you want to negotiate with (North Korean leader) Kim Jong Un, and your goal is to avoid war and try to be able to have a diplomatic resolution, the worst thing you can do is first threaten to destroy his country in the United Nations," Kerry said.

He was speaking in a private lecture delivered at Geneva's Graduate Institute.

"And secondly, screw around with the deal that has already been made because the message is don't make a deal with the United States, they won't keep their word," he said.

The nuclear deal places Iran under tough restraints, including round-the-clock surveillance and tracking every ounce of uranium produced, Kerry said. "We would notice an uptick in their enrichment, like that," he said, snapping his fingers.

"And nobody that I know of with common sense can understand what the virtue is in accelerating a confrontation with the possibility that they might decide they want to break out and make it [a nuclear bomb] now instead of 10 or 15 or 25 years from now," he said.

If Iran violated the accord, UN sanctions would snap back into place, Kerry said. "Moreover, at that point in time folks, we have a year of break-up. We have all the time that we need in the world to be able to bomb their facilities into submission."

Ending the deal could lead to Iran hiding fissile production facilities "deep in a mountain where we have no insight".

"So the scenario that Trump opens up by saying 'let's get rid of the deal' is actually proliferation, far more damaging and dangerous," Kerry said.

Iranian Supreme Leader Ayatollah Ali Khamenei said on Wednesday that Tehran would stick to its accord as long as the other signatories respected it, but would "shred" the deal if Washington pulled out, state TV reported.

Kerry also spoke of Trump’s “chaotic” tweets in an interview with Swedish broadcaster RTS. Trump has created a destructive atmosphere of "chaos politics" with his prolific use of Twitter for policy announcements and incendiary remarks alike, Kerry said.

"More and more Americans are finding the tweet phenomenon tiring, destructive and interruptive of the genuine kind of dialogue," he said.

"I think it creates a chaos politics, and that's not good," he added.

Trump has used Twitter as a platform to announce major shifts in both domestic and foreign policy, to criticise allies, taunt US adversaries and threaten North Korea.