DUBAI (Reuters) -Iran tipped off Russia about the possibility of a major "terrorist operation" on its soil ahead of the concert hall massacre near Moscow last month, three sources familiar with the matter said. In the deadliest attack inside Russia in 20 years, gunmen opened fire with automatic weapons at concertgoers on March 22 at the Crocus City Hall, killing at least 144 people in violence claimed by the Islamic State militant group. The United States had also warned Russia in advance of a likely militant Islamist attack but Moscow, deeply distrustful of Washington's intentions, played down that intelligence.