BERLIN (AP) -- The head of the U.N. atomic watchdog urged world powers Monday to continue dialogue with Iran to keep it in the landmark 2015 deal aimed at preventing the country from building nuclear weapons, and to help defuse mounting tensions in the region.

After his regular update to the International Atomic Energy Agency's board of governors in Vienna, Yukiya Amano told reporters that Tehran had increased its uranium enrichment activities as it threatened it would.

He said Iran's low-enriched uranium "production rate is increasing," but it wasn't clear yet whether it has exceeded the limitations set in the so-called Joint Comprehensive Plan of Action, or JCPOA.

The 2015 nuclear deal was designed to keep Tehran from building atomic weapons in exchange for economic incentives. But the decision by President Donald Trump to pull the U.S. out and to increase sanctions on Iran has taken a toll on the Iranian economy and left the other countries involved -- Britain, France, Germany, China and Russia -- struggling to find ways to keep the accord alive.

Amano said Tehran's decision on May 8 to give those countries 60 days to come up with a plan to shield Iran from American sanctions had heightened tensions. If no plan emerges, Tehran said it would ramp up its enrichment of uranium beyond the purity allowed under nuclear deal.

Following the May 8 announcement, Tehran said it had increased its uranium-enrichment production capacity, though only of the lower-enriched uranium permitted by the agreement.

"I am worried about increasing tensions over the Iranian nuclear issue," Amano said. "As I have constantly emphasized, the nuclear-related commitments entered into by Iran under the JCPOA represent a significant gain for nuclear verification -- I therefore hope that ways can be found to reduce current tensions through dialogue."

He would not elaborate on what kind of talks were necessary, saying that was a "political matter," and would not comment on whether he thought the U.S should return to the JCPOA itself.

In a confidential regular report distributed to member states at the end of May, the IAEA said in the past quarter, Iran had stayed within key limitations set by the deal.

At the same time, it reported that in the quarter, Iran's stockpiles of low-enriched uranium and heavy water are growing and raised questions for the first time about Iran's adherence to a key but vague provision intended to limit the country's use of advanced centrifuges.

Amano told reporters it was not the IAEA's function to say whether Iran was in compliance with the JCPOA, though it has at times where it is "crystal clear."

Since Iran's May 8 announcement, however, Amano said it was no longer appropriate for the IAEA to assess compliance, deferring instead to the JCPOA nations to make that judgment.

"I did not say Iran is implementing (the JCPOA), but I did not say Iran is not implementing either," he said.