DUBAI, United Arab Emirates—Iran on Sunday described a blackout at its underground Natanz atomic facility an act of “nuclear terrorism,” raising regional tensions as world powers and Tehran continue to negotiate over its tattered nuclear deal. While there was no immediate claim of responsibility, suspicion fell immediately on Israel, where its media nearly uniformly reported a devastating cyberattack orchestrated by the country caused the blackout. If Israel was responsible, it further heightens tensions between the two nations, already engaged in a shadow conflict across the wider Middle East. Israeli Prime Minister Benjamin Netanyahu, who met Sunday with U.S. Defense Secretary Lloyd Austin, has vowed to do everything in his power to stop the nuclear deal. Details remained few about what happened early Sunday morning at the facility, which initially was described as a blackout caused by the electrical grid feeding its above-ground workshops and underground enrichment halls. Ali Akbar Salehi, …