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

"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.

Bogomaz said the device went off "on the 136th kilometre" of the railroad between regional hub Bryansk and the town of Unecha, towards the border with Ukraine. "There were no casualties," he added.

Pictures shared on social media showed several of a train's tank carriages laying on their side and dark grey smoke billowing into the air at the site of the derailment, in the Bryansk region.

The attack 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, reported The Moscow Times.

Meanwhile, Russia's defence ministry says its forces had carried out missile strikes against Ukrainian military targets overnight, state-owned Ria news agency reported.

The defence ministry said all its designated targets, including weapons depots and ammunition factories, had been hit.

It added that Russian forces continued their advance in the city of Bakhmut.

The US ambassador to Kyiv, Bridget Brink, condemned Russia's overnight missile attack and called it "barbaric".

On Twitter, Brink said, "Russia again launched missiles in the deep of night at Ukrainian cities where civilians, including children, should be able to sleep safely and peacefully."

"I am grateful for those who protect Ukraine's skies, and the United States will continue to work hard and fast to support them and their ability to defeat Russia's barbaric attacks on the people of Ukraine," he added.