Russia on Thursday arrested a Belarusian man for allegedly placing explosives on two trains on a key part of a Siberian railway near the Chinese border, on Ukraine's orders.
The arrest came around a week after authorities reported traffic complications in a tunnel on the Baikal-Amur mainline, some 5,500 kilometres (3,400 miles) east of Moscow.
Ukrainian sources told AFP last week that Kyiv's SBU was behind the sabotage.
"On the territory of the Omsk region, a citizen of Belarus born in 1971 was arrested, for being behind explosions on November 29 and 30 on two trains carrying oil products travelling ... along the routes of the Baikal-Amur railway line in the Buryatia region," Russia's FSB security service said.
Moscow said the man's curator was a Lithuania-based Belarusian who it said worked for Ukrainian secret services.
It said the "terrorist act" was aimed at "destroying critical transport-energy infrastructure."
Russia's RIA Novosti news agency published a video of a uniformed FSB officer escorting the man, whose face is not shown, through the snow into a building.
In the video he says that someone had asked him to "bring a parcel in exchange for money" into Russia and that he was told it contained "a mine to blow up some trains."
"I agreed," he said, in the FSB-controlled video.
The powerful security force said it was working to establish if others had aided the man.
Railway sabotage has been reported inside Russia as well as its ally Belarus during Moscow's Ukraine offensive.