VIENNA (AP) ― A senior U.S. diplomat urged Iran on Wednesday to follow up on good will generated by moderate statements from its new president with actions that ease suspicions it is trying to make nuclear arms.

The tone of outreach instead of censure reflected Washington’s hopes that Tehran will seize the moment created by change in its political leadership and act to ease international tensions over its nuclear ambitions.

Still, U.S. envoy Joseph Macmanus warned of tough diplomatic action unless Tehran cooperates with U.N. experts trying to determine whether it ever worked on such weapons ― a threat echoed by a statement from the European community.

Iran insists it wants to harness the atom only to generate power or for scientific or medical purposes. But Western comments at a high-level session of the U.N.’s International Atomic Energy Agency reflected more than a decade of fears that Tehran also seeks the ability to be able to make nuclear arms.

In comments to the IAEA’s 35-nation board, Macmanus noted the “unique moment” produced by the election triumph of President Hasan Rouhani over more hard-line rivals.

At the same time, he suggested that the West will push at the IAEA’s November board meeting to punish Tehran by referring it to the U.N. Security Council unless it cooperates with IAEA experts trying to probe its alleged secret nuclear weapons work.

Iran denies having trying to develop such arms. It and the IAEA blame each other for delays in reaching agreement on a probe.

Ten rounds of negotiations over the past two years have failed to end the deadlock. The two sides meet again Sept. 27, and Macmanus indicated that the West will consider those talks a yardstick of Rouhani’s professed interest in easing nuclear tensions.

The West, he said, will work with other board members to hold Iran accountable should it fail to seize the moment and “continue its intransigence and obfuscation.”

His comments appeared to be diplomatic code for an effort in November to again refer Iran to the U.N. Security Council if the Sept. 27 talks end inconclusively.

Past referrals have led to U.N sanctions. While permanent council members Russia and China would likely veto additional sanctions, a new referral would still be a harsh international expression of displeasure with Iran.

Also voicing the threat of referral, a statement from the European Union warned of possible “action” if Iran does not cooperate with probe attempts by November.

The IAEA is particularly interested in visiting a site at Parchin, a sprawling military complex southeast of Tehran, where it suspects Iran worked on a conventional explosives trigger for a nuclear blast.

Washington and its allies also worry about Iran’s expanding uranium enrichment program and construction of a plutonium-producing reactor. Iran says both programs are only for peaceful purposes but the West fears Tehran could re-engineer them to produce the core of nuclear weapons.