A Russian freight train derailed Monday in the western region of Bryansk bordering Ukraine after an "explosive device" detonated on the rail tracks, the local governor said.

On the same day, officials said power lines had been blown up in northern Russia, which the FSB security service called an "act of sabotage."

The apparent attacks came a day after a Ukrainian strike killed four people in a Russian village in the Bryansk region and as Kyiv prepared for a widely expected counter-offensive.

"An unidentified explosive device went off, as a result of which a locomotive of a freight train derailed," Bryansk governor Alexander Bogomaz said on Telegram.

There were no casualties, he added.

He said emergency services were working at the scene and that rail traffic in the area had been suspended.

There have been reports of sabotage acts on railroads in Russia and its ally Belarus throughout Moscow's more than year-long Ukraine offensive. But this is the first time Russian officials confirm an attack of that scale.

Footage on social media showed the front of the train and several cargo carriages on fire and lying on the grass next to the tracks.

Russian Railways said the incident took place on Monday at 10:17 am local time (0717 GMT) between the town of Unecha and the village of Rassukha in the south-western corner of region -- some 100 kilometres (62 miles) from the Ukrainian border.

It said the front locomotive and seven wagons derailed "after the intervention of unauthorised persons in the work of railway transport."

"As a result of the incident, the locomotive caught fire," it said in a statement.

The state operator said fire fighters were working at the scene and that there could be delays on passenger trains from the area to Moscow.

Earlier on Monday, the governor of the northern Leningrad region, Alexander Drozdenko, said local power lines had been blown up by an "explosive device."

The official said the lines were damaged near the village of Susanino, 60 kilometres (37 miles) south of Russia's second city Saint Petersburg and posted images of the lines lying on the ground in a forest.

Drozdenko later said the FSB security service had opened a criminal case on "sabotage".