VIENNA (AP) ― Iran has allowed a top U.N. atomic inspector access to a site where it is developing advanced centrifuges that can be used to make nuclear fuel and to arm warheads, diplomats told the Associated Press on Tuesday.

The diplomats said that Deputy Director General Herman Nackaerts of the International Atomic Energy Agency also was allowed to tour Iran’s heavy water production plant for the first time. Heavy water reactors ― like the research unit being built by Iran ― produce plutonium which, along with enriched uranium, can be used for the fissile core of nuclear warheads.

A senior diplomat familiar with the visit described the Iranian move as significant in demonstrating openness after years of stonewalling IAEA requests for greater access to restricted nuclear activities.

Iran is under four sets of U.N. Security Council sanctions because it refuses to suspend both enrichment and its heavy water reactor program. Therefore any sign that it is ready to open a larger window is likely to blunt U.S.-led efforts to increase pressure on the Islamic Republic for defying the Security Council.

Western nations at the IAEA are tentatively planning to seek a resolution at the agency’s November meeting of its 35-nation board that would again report it to the Security Council ― this time for stonewalling IAEA attempts to probe its secrets, including alleged work on nuclear arms.

Before Nackaerts’ five-day visit, which ended Saturday, the IAEA was forced to rely on satellite imagery in concluding that the heavy water production plant was in operation. One of the diplomats said Nackaerts was able to confirm this on his visit.

Both the diplomat and a counterpart from a different IAEA member nation asked for anonymity because their information was privileged.

Since Iran’s secret nuclear activities were unveiled in 2003, concerns have grown that it may be using the cover of a peaceful program to develop weapons capability.

Iran denies that, saying its activities are geared only toward research and producing nuclear fuel. But because both enrichment and a running heavy water reactor can produce warhead material, international fear has grown as Tehran defies U.N. resolutions demanding a stop to both programs.

Iran’s work on advanced centrifuges has heightened worries because ― once operational ― they will be able to enrich at up to three times the speed of its present model.

To date, the IAEA has no knowledge of Iran making weapons-grade uranium. But any speed-up in enrichment would increase its supply of low-enriched material more quickly ― and also speed up conversion into high-enriched, weapons grade uranium, should Iran choose to go that route.

In its last report in May, the IAEA said that Tehran has stockpiled more than four tons of low-enriched uranium. That would be enough for nearly three nuclear warheads if enriched to levels above 90 percent.

It also is increasingly focused on higher enrichment to 20 percent ― a level that can be turned into weapons-grade uranium much more quickly than Iran’s 3.5 percent low-enriched uranium.

The diplomats said that Nackaerts also visited Iran’s underground enrichment site at Fordow which offers better protection from possible airstrikes than its present facility and which is currently being outfitted with centrifuges.

On Monday, U.S. State Department spokeswoman Victoria Nuland said Iran’s recent enrichment moves are increasingly suspicions.

“The Iranian nuclear program offers no plausible reasons for its existing enrichment of uranium up to nearly 20 percent, nor ramping up this production, nor moving centrifuges underground,” she said. “And its failure to comply with its obligations to suspend its enrichment activities up to 3.5 percent and nearly 20 percent have given all of us in the international community reason to doubt its intentions.”

The Fordow bunker facility near the holy city of Qom, Iran, is to house approximately 3,000 centrifuges. Iran’s main enrichment plant near the central city of Natanz has more than 5,000 operating machines.