LONDON — The Director General of the International Atomic Energy Agency (IAEA) Rafael Grossi said that Iran’s failure to provide credible explanations for traces of uranium found at two undeclared sites is “a big problem” that is affecting the country’s credibility.

He disclosed that IAEA experts found traces of uranium that have been subject to industrial processing in different places, which had not been declared by Iran.

"A return to the old Joint Comprehensive Plan of Action (JCPOA) was not feasible in the talks in Vienna," he said.

He added, that there is need to what he described as “an agreement within an agreement, or an implementation road map” on how to address Iran’s improved nuclear strength, including its use of more advanced centrifuges than allowed under the 2015 agreement.

The head of the United Nations nuclear watchdog has said Iran is enriching uranium to levels that only countries seeking to make atomic weapons reach, and that Iran’s nuclear program can no longer be returned to where it stood when a landmark 2015 deal was struck with world powers.

“A country enriching at 60 percent is a very serious thing — only countries making bombs are reaching this level,” IAEA chief Grossi said in an interview with the Financial Times published on Wednesday and cited by the Reuters News Agency.

“Sixty percent is almost weapons-grade, commercial enrichment is 2, 3 [percent],” he said. “This is a degree that requires a vigilant eye.”

Though Grossi conceded that Iran has the right to develop its nuclear program, he warned of the consequences of it going too far.

“You cannot put the genie back into the bottle — once you know how to do stuff, you know, and the only way to check this is through verification,” he said, referring to checks by UN monitors.

With the sophistication that Iran has achieved, “you want a really strong, very sturdy verification system,” he said.

Grossi’s remarks came as world powers resumed talks with Iran to save the 2015 nuclear agreement that limited the Iranian nuclear program in return for sanctions relief.

The US has since pulled out of the pact, applying sanctions, after which Iran began to publicly renege on its own commitment to the deal.

Iran’s violations since the US withdrawal from the deal include a significant increase in the purity and quantity of uranium it has been enriching, effectively reducing the so-called breakout time to produce an atomic bomb.

Iran says it does not want to build an atomic bomb, insisting that its nuclear program is for civilian purposes only.

Grossi predicted that even if the nuclear deal is revived, it will not be possible to return Iran’s program to its 2015 state because it has advanced so much.

“The Iranian program has grown, become more sophisticated so the linear return to 2015 is no longer possible,” Grossi said. “What you are able to do is keep their activities below the parameters of 2015.”

On Tuesday, world powers opened a fifth round of talks with Iran aimed at bringing the US back into the nuclear deal, with both sides expressing hope that it might be the final series of negotiations.

The talks in Vienna came a day after the IAEA struck a last-minute agreement with Tehran for a one-month extension to a deal on surveillance cameras at Iran’s nuclear sites.

The issue wasn’t directly related to the ongoing talks on the nuclear accord,

JCPOA, but if Iran had not agreed it could have seriously complicated the discussions.

The US is not directly involved in the talks, but an American delegation headed by US President Joe Biden’s special envoy for Iran, Rob Malley, has been in the Austrian capital.

Representatives from the other powers involved — Germany, France, Britain, Russia and China — have shuttled between the Americans and the Iranians to facilitate indirect talks. — Agencies