The Security Service of Ukraine (SBU) carried out the operation in the early hours of Thursday, the Reuters news agency reported.
SBU set off the explosives as a cargo train was moving through the Severomuysky Tunnel in Russia's Buryatia region, which borders Mongolia, a Ukrainian official said on condition of anonymity, according to Reuters.
The blasts paralysed "the only serious railroad connection between Russia and China," used by Moscow for military supplies, the Espreso TV website reported.
Russian authorities acknowledged that "a train carrying fuel caught fire in the tunnel," adding that a preliminary probe was underway to establish the cause.
An attack more than 4,000 kilometres from Ukraine would highlight Kyiv's ability to carry out operations deep inside Russia, Reuters reported.
Russia strikes Ukraine's east, south, killing three civilians
Russia on Thursday carried out a series of strikes on Ukraine's eastern Donetsk region and southern Kherson province, killing three people and injuring 12, The Kyiv Independent website reported.
One person was killed in the eastern city of Toretsk after Russian forces dropped two aerial bombs on a residential area, the regional prosecutor's office said.
Norway provides EUR 21m for Ukraine's food sector
The Norwegian government on Thursday announced EUR 21 million in new funding for Ukraine's food sector amid the Russian invasion, the Interfax-Ukraine news agency reported.
Norway will channel the funds through the United Nations World Food Programme and the World Bank, according to Foreign Minister Espen Barth Eide.
"In addition to providing food aid near the front lines, the funding will be used for demining of agricultural land, allowing farmers to go back to food production as quickly as possible," the government in Oslo said.
Russia invaded Ukraine on February 24, 2022, launching the largest military campaign in Europe since World War II.
Friday is day 646 of Russia’s war on Ukraine.
Source: Reuters, Espreso TV, Interfax-Ukraine, The Kyiv Independent