Ukraine on Monday said it had struck a base used by a Russian paramilitary group as well as a bridge near the occupied city of Melitopol, according to AFP.

Sergiy Gaiday, governor of the Lugansk region in eastern Ukraine, said the base of the Wagner group was "destroyed by a precision strike".

Little is known about the shadowy paramilitary group which is believed to be linked to Russian oligarch Yevgeny Prigozhin, who is himself a close associate of President Vladimir Putin.

The presence of Wagner paramilitaries has been documented in Libya, Mali and Syria, among many other countries -- particularly in Africa.

Ukrainian authorities also said that saboteurs had blown up a railway bridge southwest of the city of Melitopol, which is held by Russian troops.

Melitopol's mayor Ivan Fyodorov said on Telegram that the strike meant there would be "a complete absence of military trains from Crimea".

The Crimean peninsula, which was annexed by Russia in 2014, is an essential base for supplying Russian troops deployed in southern Ukraine.

Ukraine has said it is waging a counter-offensive in the south of the country, re-capturing dozens of villages and threatening to push Russian troops to the other bank of the Dnieper River.

Meanwhile in Odessa on the Black Sea, three people were killed while swimming due to the detonation of "an unidentified explosive device," said Sergiy Brachuk, a spokesman for the local authorities.

Officials have warned beachgoers not to enter the sea because of the presence of unexploded mines.

Earlier on Monday, Russian shelling of Kharkiv, Ukraine's second biggest city, killed at least one person and injured six more, a senior police official, Sergiy Bolvinov, said on Facebook.