Ukrainian media say agents from the country's security service caused an explosion in a tunnel on a railway route in Russia's Far East. The route is reportedly crucial for civilian and military transportation between Russia and China.

Media outlets in Ukraine quote sources as saying the blast occurred in a tunnel in the Russian republic of Buryatia on Wednesday as a freight train was going through. They say four explosive devices were detonated.

The site is about 5,000 kilometers from the frontline between Ukrainian and Russian troops.

Russia's state-run railway company said a fuel tank the train was carrying caught fire in the tunnel and local authorities are investigating the cause.

Ukraine media say the route is used to transport military supplies for Russian forces.

Meanwhile, Ukrainian President Volodymyr Zelenskyy on Thursday visited the Kupiansk front in the eastern region of Kharkiv, where he encouraged his soldiers in the area that has seen fierce fighting.

General Valerii Zaluzhnyi, the commander-in-chief of the Ukrainian forces, held telephone talks that day with General Charles Q. Brown, the chairman of the US Joint Chiefs of Staff. Zaluzhnyi said he explained the situation in Kupiansk, Bakhmut, Avdiivka and elsewhere. He also said they discussed military support, including for mine clearance and Ukraine's air force.