A US withdrawal from the Iran nuclear deal would be a new affront to the United Nations - 14 years after the invasion of Iraq, former UN weapons inspector Hans Blix warned Monday.

US President Donald Trump is a fierce critic of the 2015 accord, calling it "the worst deal ever", and he is expected to announce this week that he is "decertifying" Iran's compliance with it.

"If Mr Trump cares for the authority of the United Nations then he cannot pull back unilaterally from that agreement," Blix said in Paris.

The Swedish former diplomat headed the inspection team that found no proof of weapons of mass destruction in Iraq in 2002-03 as the United States was poised to invade the country.

Then president George W. Bush and his government insisted that Iraqi dictator Saddam Hussein had chemical and biological weapons and was developing nuclear arms.

Blix was speaking on the sidelines of a two-day International Luxembourg Forum on Preventing Nuclear Catastrophe that started in Paris on Monday.

"I am a bit puzzled by Trump's lonely attitude... because the president has voiced his great disapproval" of the Iran deal in the face of "almost the unanimous international community standing in favour of it", he said.

The nuclear agreement was struck in July 2015 by Iran and five permanent members of the UN Security Council (Britain, China, France, Russia and the United States) plus Germany - establishing controls to prevent Tehran from developing an atomic bomb.

International Atomic Energy Agency (IAEA) chief Yukiya Amano, speaking in Rome on Monday, affirmed Iran's commitment to the nuclear deal.

But Trump has said Tehran is not living up to the "spirit" of the agreement.

US officials insist "decertifying" Iran's compliance would not sink the deal itself but open the way for Congress to possibly develop new measures against Tehran.

Blix, who headed the IAEA from 1981 to 1987, led the UN Monitoring, Verification and Inspection Commission from 2000 until the eve of the US-led invasion of Iraq in March 2003.

Source: France 24