The UN watchdog policing Iran's nuclear deal with major powers admonished Tehran on Tuesday for failing to answer its questions about past nuclear activities at three sites and for denying it access to two of them. 

The International Atomic Energy Agency (IAEA) issued a second report in addition to its regular quarterly update on Iran's nuclear activities, rebuking Iran for less than full cooperation in general and for failing to grant UN inspectors access to one or more sites of interest. 

The regular report showed Iran's stockpile of low-enriched uranium nearly tripling since November to more than a ton, as Tehran continues to breach limits of its 2015 nuclear deal in response to renewed U.S. sanctions against it since Washington pulled out of the accord in May 2018. 

The extraordinary second report delved into the IAEA's open questions and Iran's denial of access to sites which two senior diplomats said are believed to have been active in the early 2000s. 

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"I judged it necessary to produce a second report because I thought the situation is serious enough to merit such a move on my part," new IAEA chief Rafael Grossi, who took office in December, told Reuters in an interview in Paris.

The second confidential IAEA report to member states seen by Reuters said Iran has not provided access to the agency to two locations, and not engaged in substantive discussions to clarify agency questions related to possible undeclared nuclear material and nuclear-related activities. 

"We have insisted and despite all our efforts we have not been able to get that, so the situation requires on my part such a step because what this means is that Iran is curtailing the ability of the agency to do its work," he said, adding that he hoped Tehran would reverse course after the IAEA's board meets next week in Vienna. 

What exactly is thought to have happened at the three sites, none of which the IAEA has visited before, is unclear.  

The second report said the IAEA's open questions include whether natural uranium was used at one of the three sites. At one of the other two, the agency has seen activities "from early July 2019 onwards that were consistent with efforts to sanitize part of the location," it said. 

Tuesday's second report also said Iran had informed the IAEA that it "will not recognize any allegation on past activities and does not consider itself obliged to respond to such allegations." 

Grossi warned that Iran risks triggering a new crisis if it does not cooperate with the UN nuclear watchdog. 

"I sincerely hope that Iran will listen to us and listen to the voice of the international community at the board of governors and assess that it is in their own interest to cooperate with us," he said. "We don't have a political agenda; we simply are requesting them to comply with their obligations." 

"We will be walking towards a crisis (if not)," he added.

The 2015 nuclear deal, which lifted international sanctions against Tehran in exchange for limits on its uranium enrichment and other nuclear activities, is aimed at keeping Tehran at least a year away from accumulating enough fissile material for an atomic bomb if it sought one. 

Iran denies ever having had a nuclear arms program and says it would never seek to build an atomic bomb. 

Tensions between Tehran and Washington escalated sharply after the U.S. killing of top Iranian military commander Qasem Soleimani on January 3. Iran then announced it would abandon limitations on enriching uranium, taking a further step back from commitments to the deal. 

France, Britain and Germany confirmed in mid-January that they had triggered the dispute mechanism in the deal. 

China on Wednesday urged the IAEA to stay neutral on the investigation, saying Iran's nuclear facilities have been under the IAEA inspections and the country has not used the nuclear material for other purposes. 

China encourages Iran to continue to engage in talks with the IAEA and also hopes the IAEA could stay neutral on the issue, Foreign Ministry spokesman Zhao Lijian said in a daily briefing.

Zhao said the unilateral withdrawal by the U.S. from the JCPOA is the root cause of the tension and the U.S. should go back to dialogue and negotiation.

(With input from Reuters)