KYIV, Ukraine — Ukraine’s spy agency staged two successive explosions on a railroad line in Siberia that serves as a key conduit for trade between Russia and China, Ukrainian media reported Friday. The attacks underscored Moscow’s vulnerability amid the war in Ukraine

Ukrainska Pravda and other news outlets claimed the Security Service of Ukraine conducted a special operation to blow up trains loaded with fuel on the Baikal-Amur Mainline, which runs from southeastern Siberia to the Pacific Ocean in the Russian Far East.

The media cited unidentified sources in Ukrainian law enforcement agencies, a regular practice in claims of previous attacks in Russia. The security service, which is known in Ukrainian as SBU for short, has not confirmed the reports.

The first explosion hit a tanker train in the Severonomuisky tunnel in Buryatia early Thursday, causing a fire that took hours to extinguish, Russian news outlets said. The 15.3-kilometer (9.5-mile) tunnel in southern Siberia is the longest in Russia.

A second explosion hours later hit another train carrying fuel as it crossed a 35-meter (115-foot) high bridge across a deep gorge while traveling on a bypass route, according to the Ukrainian news reports.

Russian railways confirmed the tunnel explosion but didn’t say what caused it.

Russian daily business newspaper Kommersant cited investigators saying an explosive device was planted under one of the train’s carriages.

There was no comment from Russian authorities on the second explosion.

Ukrainian authorities have emphasized that the country’s military and security agencies can strike targets anywhere in Russia to fight Moscow’s aggression.

Officials in Kyiv have claimed responsibility for some previous attacks on infrastructure facilities deep inside Russia.

Russia’s top counterintelligence agency, the Federal Security Service, or FSB, said Friday that it detained a man accused of attacking a military airbase in western Russia with exploding drones in July and staging an explosion that derailed a cargo train in western Russia last month.

The FSB identified the suspect as a dual Russian-Italian citizen and alleged he was recruited by the Ukrainian military intelligence in Istanbul and underwent training in Latvia before returning to Russia.

There was no immediate comment on the claim from Ukrainian authorities.

As the war continued into its 22nd month, Ukraine’s forces shot down 18 of 25 Iranian-made Shahed exploding drones and one of two air-launched missiles that Russia launched early Friday, the Ukrainian air force said.

The office of Ukrainian President Volodymyr Zelenskyy reported that Russian strikes across Ukraine killed at least four civilians and wounded 16 others between Thursday and Friday mornings.

Three of them died when Russian warplanes struck the village of Sadove in southern Ukraine’s Kherson region; the fourth was killed in Russian shelling of the town of Toretsk in the eastern Donetsk region, the presidential office said.