Ukraine’s security service blew up a railway connection linking Russia to China, in a clandestine strike carried out deep into enemy territory, with pro-Kremlin media reporting that investigators have opened a criminal case into a “terrorist attack.”

The SBU set off several explosions inside the Severomuysky tunnel of the Baikal-Amur highway in Buryatia, located some 6,000 kilometers east of Ukraine, a senior Ukrainian official with direct knowledge of the operation told POLITICO.

“This is the only serious railway connection between the Russian Federation and China. And currently, this route, which Russia uses, including for military supplies, is paralyzed,” the official said.

Four explosive devices went off while a cargo train was moving inside the tunnel. “Now the (Russian) Federal Security Service is working on the spot, the railway workers are unsuccessfully trying to minimize the consequences of the SBU special operation,” the Ukrainian official added.

Ukraine’s security service has not publicly confirmed the attack. Russia has also so far not confirmed the sabotage.