Iran is stifling a United Nations probe of its alleged storage of nuclear equipment and radioactive material in Tehran, diplomats say, leading to fresh concerns about Iran’s activities at a critical moment for the fate of the 2015 nuclear deal.

The diplomats said Iran has refused to provide answers to important questions raised by the U.N.’s International Atomic Energy Agency over allegations, first made public by Israeli Prime Minister Benjamin Netanyahu a year ago, that Iran had established a now-dismantled site in Tehran to store equipment and material used during past nuclear weapons work.

It is the first time Iran seems to have refused to cooperate with the IAEA’s monitoring of its activities since the multination accord was implemented in January 2016. It comes as European powers, led by France, try to prevent the deal’s collapse after the U.S. withdrew from the agreement in May 2018 and then reimposed tight sanctions on Iran, in violation of the deal. Until recently, the IAEA repeatedly said Iran was meeting all its commitments and cooperating with inspections.

Iranian officials in New York did not respond to a request for comment. Iran dismissed Mr. Netanyahu’s allegations about the site last year.

Iran’s behavior has sparked sharp debate in and around the IAEA, the diplomats said. The agency, they said, had recently told member states it would criticize Iran for not cooperating. But in a report released to member states on Friday, the agency made only a vague reference to the issue.