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Ukraine's security service has blown up a rail connection deep inside Russia, a senior official has told the ����ý.

Successive explosions are said to have happened on trains running first through a tunnel and then on a bridge in Russia's far east.

Russia has reportedly begun an inquiry into a "criminal case of terrorism" after the attacks on the Baikal Amur line running to the border with China.

The extent of the damage has not been verified by the ����ý.

Ukrainian sources said that the intention was to "disable" an important piece of infrastructure that the Russians sometimes use for military purposes.

After the first incident in the Severomuysky tunnel, on Wednesday evening, Russian railway officials said that crew had noticed smoke in a fuel tank and the train was stopped.

At 15km, Severnomuysky is the longest railway tunnel in Russia and lies in the region of Buryatia, north of Mongolia.

A senior Ukrainian official told the ����ý that the freight train had in fact exploded in the tunnel.

The Russians then fell into the "trap" - claimed the official - of using an alternative bridge route.

"This is exactly what the SBU (Security Service of Ukraine) was counting on," said the official.

"During the passage of the train on this 35m-high bridge, explosive devices planted in it went off."

The second explosion has not been corroborated elsewhere although Russian telegram channel Baza has reported that another train was later blown up in the same area.

The incidents appear to have happened on Wednesday and Thursday, with no known casualties.

According to reports, the explosions have caused damage to fuel tanks and rail lines.

There's been no official or public comment from the Russian government.