The head of the United Nations’ atomic watchdog arrived in Iran on Monday, where his agency faces increasing difficulty in monitoring the Islamic Republic’s rapidly advancing nuclear programme as tensions remain high in the wider Middle East over Israel’s war on Gaza. Rafael Mariano Grossi already has warned Tehran has enough uranium enriched to near-weapons-grade levels to make “several” nuclear bombs if it chose to do so. He has acknowledged the agency can’t guarantee that none of Iran’s centrifuges may have been peeled away for clandestine enrichment . Those challenges now find themselves e…