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, Reuters said on April 1. 

"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," said one source.  

The Telegraph reported yesterday that Iran arrested 35 people in January, including a commander of the Islamic State (IS) terror group’s Afghanistan-based branch IIS-Khorasan, or IS-K, who it said were linked to twin bombings in the city of Kerman on January 3, in which almost 100 people were killed.

A second source, who reportedly also requested anonymity, said the information Tehran provided to Moscow about an impending attack had lacked specific details regarding timing and the exact target.

“They [the members of IS-K] were instructed to prepare for a significant operation in Russia... One of the terrorists [arrested in Iran] said some members of the group had already travelled to Russia,” said the source, according to The Telegraph.

A third source, a senior security official, reportedly said: “As Iran has been a victim of terror attacks for years, Iranian authorities fulfilled their obligation to alert Moscow based on information acquired from those arrested terrorists.”

According to The Telegraph, the warning echoed a similar one from the United States, which publicly warned of a possible terrorist attack in Moscow two weeks beforehand and said it had shared its intelligence with Russia.

IS claimed responsibility for the Crocus City massacre via its official Amaq news agency. The US has also attributed the attack to IS.

Meanwhile, Kremlin spokesman Dmitry Peskov said he had no information about reports of a warning from Iran.

Russia, which has arrested at least twelve suspects, including the four suspected gunmen, has accused Ukraine, the US and Britain of orchestrating the attack.