The International Atomic Energy Agency has urged Iran to resume talks “now” or risk making it “extremely more difficult” to repair the 2015 nuclear pact.
Last week, Iran disconnected 27 cameras that allowed the agency's inspectors to monitor its nuclear activities.
It was in response to a western resolution passed on June 8 in which the IAEA criticised Tehran's lack of co-operation.
In a CNN interview on Sunday, IAEA director general Rafael Grossi called the removal of the cameras a “very serious move”.
“Recent history tells us that it is never a good thing to start saying to international inspectors, go home … things get much more problematic,” he said.
The 2015 deal, known as the Joint Comprehensive Plan of Action, gave Iran relief from economic sanctions in exchange for curbs on its nuclear activities.
But in 2018, then-US president Donald Trump abandoned the pact and reimposed sanctions, prompting Iran to begin ignoring its commitments.
Talks to revive the deal stalled in March.
“We have to sit down now, we have to redress the situation, we have to continue working together,” Mr Grossi told CNN he was telling his Iranian counterparts.
“The only way for Iran to get the confidence, the trust they so badly need in order to move their economy forward … is to allow the inspectors of the IAEA to be present.”
Without the surveillance cameras, Mr Grossi said, his agency will soon be unable to declare whether the Iranian nuclear programme is peaceful, as Tehran has repeatedly said, or whether Iran is developing an atom bomb.
Even if the Iranians reconnect the cameras in a few months, Mr Grossi said any work they do in the meantime will remain secret, possibly rendering any agreement useless.
He said that makes “the way back to an agreement extremely more difficult”.
Biden ready to embrace deal
While Mr Trump pulled the US out of what he said was a flawed accord, his successor Joe Biden said he is ready to embrace the deal so long as Iran respects its commitments.
Negotiators have been repeatedly frustrated, and failure appears close.
In a phone call on Saturday with Iranian Foreign Minister Hossein Amirabdollahian, UN Secretary-General Antonio Guterres asked diplomats to rescue the agreement, according to a statement.
