VIENNA (Reuters) – Setting Iran deadlines to explain how uranium particles were found in an undeclared Tehran warehouse could be counter-productive, the new U.N. nuclear watchdog chief said on Tuesday, hoping fresh dialogue will resolve the months-long standoff. Rafael Grossi, a 58-year-old career diplomat from Argentina, took over as director general of the International Atomic Energy Agency on Tuesday. Arguably the biggest challenge he and his agency face is policing Iran’s nuclear deal with major powers. Tehran has so far failed to provide a satisfactory explanation for the uranium traces found by IAEA inspectors at a site that Israeli Prime Minister Benjamin Netanyahu pointed them to, calling it a “secret atomic warehouse”. In September the IAEA’s acting chief told Iran that “time is of the essence”, but there has been no sign of improved cooperation by Tehran since then. “To put deadlines might not be the best idea,” Grossi told Reuters in an interview when asked how long he was willing to give Iran to provide an explanation that holds water on the particles of uranium that was processed but not enriched. “This would for me mean that we would be in a very … antagonistic relationship where basically one side would be resisting and then I as DG would need to be putting deadlines,” he said, adding: “We need to work together … Time is always of the essence.” Grossi, a veteran of nuclear diplomacy who worked as a senior IAEA official from 2010 to 2013, has said he already knows some of the main decision-makers in Iran, but he has yet to meet senior Iranian officials since taking office. NEW KID ON THE BLOCK He hopes he can help set a new tone with the Islamic Republic in his first face-to-face talks, starting this week on the sidelines of a meeting in Vienna of parties to Iran’s landmark 2015 nuclear deal with world powers. Iran’s delegation is usually led by Deputy Foreign Minister Abbas Araqchi. “I’m the new kid on the block in this relationship. They’ve been there, now they get a new DG, so we have to sit down together, start talking and take it from there,” he said. “Let me start my conversation with Iran. I don’t think it would be appropriate, and it would be unfair, to pronounce myself about their attitudes before I sit down with them.” Grossi has said he will be “firm but fair” on inspections generally, including in Iran, without spelling out what that means. He told Reuters he is satisfied with the work the IAEA’s inspections team has been doing. While he has said he plans to communicate more actively than his late predecessor Yukiya Amano, he said that, like Amano, he would not voice an opinion on the state of the nuclear deal, which has been eroding as Iran breaches the limits it imposes on its nuclear activities one by one, in response to a U.S. pullout from the accord last year and reimposition of sanctions on Iran. Instead, the IAEA will stick to providing technical updates on Iran’s nuclear activities. Grossi will be more vocal on other fronts, such as the role nuclear energy can play in reducing fossil fuel emissions. He plans to travel to U.N. climate change talks in Madrid this month.