The United States is not seeking to reopen or renegotiate the Iran nuclear deal but hopes to stay in it to fix its flaws with a supplementary agreement, U.S. non-proliferation envoy Christopher Ford said on Wednesday.

"We are not aiming to renegotiate the JCPOA or reopen it or change its terms," Ford told reporters on the sidelines of a nuclear non-proliferation conference in Geneva.

"We are seeking a supplemental agreement that would in some fashion layer upon it a series of additional rules - restrictions, terms, parameters, whatever you want to call it - that help answer these challenges more effectively."

U.S. President Donald Trump and French President Emmanuel Macron pledged on Tuesday to seek stronger measures to contain Iran, but Trump refrained from committing to staying in a 2015 nuclear deal and threatened Tehran with retaliation if it restarted its nuclear programme.

Macron said he spoke to Trump about a "new deal" in which the United States and Europe would tackle the outstanding concerns about Iran beyond its nuclear programme.

Under Macron's proposal, the United States and Europe would agree to block any Iranian nuclear activity until 2025 and beyond, address Iran's ballistic missile program and generate conditions for a political solution to contain Iran in Yemen, Syria, Iraq and Lebanon.

"If we were able to meet that challenge of bringing our partners together ... President Trump made it clear that his decision not to renew the sanctions waivers would be revised, and that is where I hope we are now today," Ford said.

Asked if Macron had saved the JCPOA in talks with Trump, Ford said: "I hope the JCPOA has been saved in the context of the challenge that President Trump set for us, of trying to remain within the deal but in the context of moving forward with our partners on an approach that stands a pretty good chance of turning what was in effect a temporary postponement ... into a more enduring answer."