Russia says its forces are edging closer to capturing a salt-mining town in eastern Ukraine, which would mark an elusive victory for the Kremlin but come at the cost of heavy Russian casualties and extensive destruction of the territory they claim.

Key points:

• Ukraine says more than 100 Russian troops were killed in the battle for Soledar over the past 24 hours
• Russia has stopped short of claiming victory over the town, as had been done earlier by the Wagner mercenary group
• Elsewhere in Ukraine, at least four civilians have been killed and 16 injured in fighting across the nation

More than 100 Russian troops were killed in the battle for Soledar over the past 24 hours, Ukraine's Donetsk Governor Pavlo Kyrylenko said in televised remarks.

"The Russians have literally marched on the bodies of their own soldiers, burning everything on their way," Mr Kyrylenko said while reporting that Russian forces had shelled a dozen towns and villages in the region in the past day.

Ukraine's Deputy Defence Minister Hanna Malyar said fighting was still fierce and that the Russians were "moving over their own corpses".

The Special Operations Forces of the Armed Forces of Ukraine said in a statement earlier on Thursday that it had successfully targeted a large concentration of Russian soldiers in an area of Soledar.

"As a result of these several strikes, more than 100 invaders ... were confirmed to be destroyed," the statement said.

Russian forces are using mortars and rockets to bombard Soledar in an unrelenting assault, struggling for a breakthrough after military setbacks have turned what the Kremlin hoped would be a fast victory into a grinding war of attrition that has dragged on for nearly 11 months with no end in sight.

"Civilians are trying to survive amid that bloodbath as the Russians are pressing their attacks," Mr Kyrylenko said.

Serhii Cherevatiy, a spokesman for Ukraine's forces in the east, said Soledar was hit by Russian artillery more than 90 times in the past day.

Soledar's fall would be a prize for a Kremlin starved of good battlefield news in recent months, after losing the significant city of Kherson in December.

It would also offer Russian troops a springboard to conquer other areas of the eastern Donetsk province that remain under Ukrainian control, particularly the nearby strategic city of Bakhmut.

Wagner mercenaries take heavy hits in Russian battlefield tactics:

The Russians' tactic in the assault on Soledar is to send one or two waves of soldiers, many from the private Russian military contractor Wagner Group, who take heavy casualties as they probe the Ukrainian defences, a Ukrainian officer near Soledar told The Associated Press.

When Ukrainian troops suffer casualties and are exhausted, the Russians send in another wave of highly-trained soldiers, paratroopers or special forces, to get a new toehold on the battlefield, said the Ukrainian officer, who insisted on anonymity for security reasons.