Employees of Ukraine’s special services (SBU) blew up a railway line in Siberia that Russia uses for military supplies, a Ukrainian source told Reuters on Thursday.

The source, who insisted on anonymity, said the four explosive devices went off between Wednesday night and Thursday as the freight train passed through the Pivnichomui Tunnel in Buryatia, which borders Mongolia.

Such an attack more than 4,000 kilometers from Ukraine would be another serious blow to the SBU and a demonstration of its ability to conduct operations anywhere in Russia.

Reuters was unable to independently verify this information or determine whether the rail line is actually being used for military supplies. Russian sources acknowledged that a train caught fire in the area, but did not mention explosives.

The message of the Investigative Committee of the Russian Federation states that a train carrying fuel caught fire in the tunnel on Wednesday evening, there were no casualties, and a preliminary investigation is underway to determine the cause.

The Russian Railways reported that trains heading through the tunnel were rerouted and traffic was not impeded.