Earlier in the day, Kremlin spokesman Dmitry Peskov said he had no information about Iran's alleged advance warning.
"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 was quoted as saying, referring to the January 3 terrorist attack in the city of Kerman, which US intelligence sources attributed to the Afghanistan-based offshoot of the Daesh* terrorist group — ISIS-Khorasan*.
The tip-off did not contain any details on the exact timing and targets of the impending terrorist attack, the media reported, citing a second source.
"They [the members of ISIS-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," the same source said.
A third source, an unnamed senior security official, was cited as saying that Iranian authorities had "fulfilled their obligation to alert Moscow" based on information garnered from those detained after the Kerman bombings.
The attacks in Kerman and near Moscow both allegedly involved Tajik nationals, the media reported, adding that Tehran had discussed with Dushanbe its concern over the increased involvement of ethnic Tajiks in militant acts.
On March 22, several armed men broke into Crocus City Hall and started shooting at people. They also started a fire in one of the auditoriums, which was full of people ahead of a concert. The attack left 695 casualties, including 144 dead, according to the latest data from the Russian Emergencies Ministry. The four main suspects in the case — all citizens of Tajikistan — tried to flee the scene in a car but were detained and charged with terrorism. Russian authorities believe their plan was to flee to Ukraine, where the masterminds of the attack had arranged a safe haven for them. An investigation is underway.
The US Embassy in Moscow said in a security alert in early March that unspecified reports suggested that extremists had imminent plans to target large gatherings in Moscow, including concerts. Russian Deputy Foreign Minister Sergey Ryabkov said at the time that the United States had not informed Russia about a possible terrorist threat in Moscow.
*Terrorist organizations banned in Russia and many other countries.