Ukraine's State Bureau of Investigation said that according to its preliminary findings, "on March 18, a shell detonated during routine firing (drills) carried out by a military unit in the Chernigiv region".

"The explosion killed four soldiers," it said.

Investigators have opened a probe into possible violations of rules on handling weapons.

The Chernigiv region, an area the size of Belgium, borders both Belarus and Russia to the north.

It was partially taken by the Russian army in the first few weeks of the invasion launched in February 2022, before its forces withdrew in spring last year.

Ukraine is closely monitoring the movements of Russian troops stationed nearby, but it does not expect a fresh attack for now.