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, Paralel.Az reports citing Reuters.

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.

It is harder, however, for Russia to dismiss intelligence from diplomatic ally Iran on the attack, which has also raised questions over the effectiveness of Russian security services. Moscow and Tehran, both under Western sanctions, have deepened military and other cooperation during the two-year Ukraine war.

"Days before the attack in Russia, Tehran shared information with Moscow about a possible big terrorist attack inside Russia that was acquired during interrogations of those arrested in connection with deadly bombings in Iran," one of the sources told Reuters.

Iran arrested 35 people in January, including a commander of Islamic State's Afghanistan-based branch ISIS-Khorasan (ISIS-K), who it said were linked to twin bombings on Jan. 3 in the city of Kerman that killed nearly 100 people.

Islamic State claimed responsibility for the Iran blasts, the bloodiest since the 1979 Islamic Revolution. U.S. intelligence sources said ISIS-K had carried out both the Jan. 3 attacks in Iran and the March 22 shootings in Moscow.

Islamic State once occupied large swathes of Iraq and Syria, imposing a reign of terror and inspiring lone wolf attacks in Western countries, but was declared territorially defeated in 2017.

However ISIS-K, one of its most fearsome branches, has raised the group's profile again with large-scale bloodshed.

ISIS-K, named after an old term for a region that encompassed parts of Iran, Turkmenistan and Afghanistan, emerged in eastern Afghanistan in late 2014 and quickly established a reputation for extreme brutality.