The director of the International Atomic Energy Agency (IAEA) has reiterated concern over Iran’s explanation of uranium traces in sites used two decades ago.

In an interview with Dubai-based Al-Arabiya published Saturday, Rafael Mariano Grossi called on Iran to improve cooperation with the agency, which Tehran has reduced to that required under the Nuclear Non-Proliferation treaty.

Some analysts have linked what Grossi called “undeclared nuclear material” to Tehran’s past cooperation over centrifuges − devices used for enrichment − with Pakistani scientist AQ Khan.

Grossi said talks between Iran and world powers in Vienna to revive the 2015 nuclear deal, the JCPOA (Joint Comprehensive Plan of Action), were headed in the right direction but faced challenges.

“We must watch closely and be patient to reach a diplomatic solution,” Grossi said, noting that success hinged upon “what the political parties sitting at the negotiating table want.”

Under the JCPOA, the IAEA had extensive access to Iran’s nuclear program beyond that required under the NPT and the Additional Protocol that Iran first implemented 2003.

Iran reduced IAEA access to its nuclear sites in early 2021, when it boosted uranium enrichment demanding the lifting of US sanctions.

Grossi has reached an ad hoc arrangement with Tehran over access, which he says would allow him a full picture of the program if and when the JCPOA is back in place.