FILE - In this June 6, 2018, file frame from Islamic Republic Iran Broadcasting, IRIB, state-run TV, three versions of domestically-built centrifuges are shown in a live TV program from Natanz, an Iranian uranium enrichment plant, in Iran. A report Friday, May 31, 2019, by U.N. nuclear watchdogs said Iran had begun installing IR-6s like the one shown on the left. That raised questions for the first time about its adherence to a key provision of Iran’s 2015 nuclear deal with world powers that was intended to limit the country’s use of advanced centrifuges. (IRIB via AP, File)

FILE - In this June 6, 2018, file frame from Islamic Republic Iran Broadcasting, IRIB, state-run TV, three versions of domestically-built centrifuges are shown in a live TV program from Natanz, an Iranian uranium enrichment plant, in Iran. A report Friday, May 31, 2019, by U.N. nuclear watchdogs said Iran had begun installing IR-6s like the one shown on the left. That raised questions for the first time about its adherence to a key provision of Iran’s 2015 nuclear deal with world powers that was intended to limit the country’s use of advanced centrifuges. (IRIB via AP, File)

VIENNA (AP) — The U.N. atomic watchdog said Friday that Iran continues to stay within the limitations set by the 2015 nuclear deal with major powers, but reported its 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.

In a confidential quarterly report distributed to member states and seen by The Associated Press, the International Atomic Energy Agency said Iran has stayed within key limitations set in the so-called Joint Comprehensive Plan of Action, or JCPOA, for uranium and heavy water stockpiles.

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But while in past reports the IAEA said Iran’s research and development on enrichment “has been conducted using centrifuges within the limits defined in the JCPOA,” the Friday report instead changed the wording to say it “has been conducted using centrifuges specified in the JCPOA.”

A centrifuge is a device that enriches uranium by rapidly spinning uranium hexafluoride gas. Under the atomic accord, Iran has been limited to operating 5,060 older-model IR-1 centrifuges.

In a footnote, the agency said that “up to 33” more advanced IR-6 centrifuges have been installed and that “technical discussions in relation to the IR-6 centrifuges are ongoing.”

Under terms of the nuclear deal, Iran is allowed to test no more than 30 of the IR-6s once the deal has been in place for 8 1/2 years. The deal is murky about limits before that point, which will arrive in 2023.

The head of the Atomic Energy Organization of Iran, Ali Akbar Salehi, said last month that Iran had begun installing a chain of 20 IR-6 centrifuges at its underground Natanz enrichment facility. Iranian officials say the IR-6 can enrich 10 times faster than an IR-1.

Iran maintains that it is allowed to install the centrifuges, regardless of the agreement’s limit on their use for testing. A senior diplomat who spoke on condition of anonymity because he wasn’t officially allowed to comment on the report said the faster centrifuges were not yet being fed with uranium.

The diplomat said the technical discussions on the centrifuges were between the deal’s signatories and Iran, but would not elaborate. “It is being discussed, and we report the facts that we see,” the diplomat said.

“The feed line is under agency seal,” the diplomat said, adding it was up to partner countries in the deal to determine whether the installation was a violation of the accord.

The nuclear deal is meant to keep Tehran from building atomic weapons in exchange for economic incentives. It has been complicated by the unilateral withdrawal of the United States from the deal last year and Washington’s increased sanctions, which has been taking a toll on the Iranian economy.

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That has left the other signatories — Germany, Britain, France, Russia and China — struggling to come up with enough incentives to keep Iran in the deal.

Earlier this month, Iran announced that if a way couldn’t be found within 60 days to shield it from U.S. sanctions targeting its economy and oil industry, it would ramp up its enrichment of uranium beyond the purity allowed under nuclear deal. And about a week ago, Iran said it had increased its uranium-enrichment production capacity, though only of the lower-enriched uranium permitted by the agreement.

In its first quarterly report since those announcements, however, the Vienna-based IAEA found Iran continued to be in compliance and also said its inspectors had been given unfettered access to Iranian nuclear facilities.

“Timely and proactive cooperation by Iran in providing such access facilitates implementation of the additional protocol and enhances confidence,” the report stated, referring to the procedure detailing safeguards and tools for verification.

The senior diplomat said Iran does have the capacity to quadruple uranium enrichment as it recently threatened, but that inspectors would have to wait until the next report to determine whether they had actually set that increase in motion.

“They have the flexibility, they can increase and they can reduce, and they can do a number of things,” the diplomat said. “The capacity is always there, and we do verify this at a technical level, we are fully monitoring that.”

Earlier this month, the U.S. ended deals allowing Iran to exchange its enriched uranium for unrefined yellowcake uranium with Russia, and to sell its heavy water, which is used as a coolant in nuclear reactors, to Oman. That will also make it difficult, if not impossible, for it to stay within stockpile limits if it increases production of both.

The IAEA said Iran’s heavy water stockpile was 125.2 metric tons (138 U.S. tons) as of May 26, up from 124.8 tons in February but below the 130 ton limit. Its stock of low-enriched uranium was 174.1 kilograms (383.8 pounds) as of May 20, up from 163.8 kilograms in February; the limit is 202.8 kilograms.

It added that Iran had not enriched any uranium above the level allowed by the JCPOA.

“All centrifuges and associated infrastructure in storage have remained under continuous agency monitoring,” the IAEA said.

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Associated Press writers David Rising in Berlin; Jon Gambrell in Dubai, United Arab Emirates; and Amir Vahdat in Tehran, Iran, contributed to this report.