Ukrainian troops, on the defensive for four months, will launch a long-awaited counterassault "very soon" now that Russia's huge winter offensive is losing steam without taking Bakhmut, Ukraine's top ground forces commander said on Thursday.

The remarks by Colonel-General Oleksandr Syrskyi were the strongest indication yet from Kyiv that it is close to shifting tactics, having absorbed Russia's onslaught through a brutal winter.

Russia's Wagner mercenaries, trying to capture Bakhmut in what has become the longest and bloodiest battle of the war, "are losing considerable strength and are running out of steam," Syrskyi said on the Telegram social media site.

"Very soon, we will take advantage of this opportunity, as we did in the past near Kyiv, Kharkiv, Balakliya and Kupiansk," he said, listing Ukrainian counteroffensives last year that proved turning points in the war, recapturing swathes of land.

Syrskyi was one of the top commanders behind Ukraine's strategy last year that repelled Russia's assault on Kyiv and rolled back Moscow's forces through the second half of 2022.

But front lines in Ukraine have largely been frozen in place since Ukraine's last major offensive in November. Since then, Moscow has sent hundreds of thousands of freshly called-up reservists and convicts recruited from prisons into battles that both sides describe as a meat grinder.

The Russian campaign has yielded few gains, and Ukraine, which had looked likely to pull out of the small eastern city of Bakhmut, decided this month to keep its troops there, denying Moscow its first victory since last August.

Kyiv has long said it plans a major counteroffensive at some point this year, using newly supplied Western arms. Several of its most successful offensives last year followed quickly after Russia had exhausted its forces in huge battles in the east.

There was no immediate response from Moscow to the latest claims its forces in Bakhmut were losing momentum, but Yevgeny Prigozhin, the Wagner mercenary boss, has issued pessimistic statements in recent days warning of a Ukrainian counterassault.

On Monday Prigozhin published a letter to Defence Minister Sergei Shoigu, saying Ukraine aimed to cut off Wagner's forces from Russia's regular troops, demanding Shoigu act to prevent this and warning of "negative consequences" if he failed.

On Wednesday, Britain's defense ministry reported that Ukraine had launched a local counterattack west of Bakhmut that was likely to relieve pressure on the main route used to supply Kyiv's forces inside the city.

There was still a threat that Ukrainian forces in Bakhmut could be surrounded, it said, but there was "a realistic possibility the Russian assault on the town is losing the limited momentum it had obtained."

This week, President Vladimir Putin made his grandest diplomatic gesture since launching the war a year ago, hosting Chinese President Xi Jinping in Moscow for a three-day state visit. The two leaders pledged friendship and jointly denounced the West, but Xi barely mentioned the Ukraine war in public.

On Wednesday, the day Xi left, Moscow sent a swarm of drones to conduct air strikes across northern Ukraine and rockets hit two apartment blocks in Zaporizhzhia in the south.

The death toll rose on Thursday to nine from one of those attacks, a dormitory struck in a riverside town south of Kyiv.

Russia invaded Ukraine in February last year in what it calls a "special military operation," claiming that Kyiv's close ties to the West were a security threat. Since then, tens of thousands of Ukrainian civilians and soldiers on both sides have been killed. Russia has destroyed Ukrainian cities and set millions of people to flight. It claims to have annexed nearly a fifth of Ukraine.

Kyiv and the West call the war an unprovoked assault to subdue an independent country.

Last week, the International Criminal Court issued an arrest warrant for Putin on war crimes charges, accusing him of illegally deporting Ukrainian children. Moscow denies this and says it has taken in children to protect them.

Dmitry Medvedev, a Putin ally who stood in for him for four years as president when Putin took the role of prime minister, said arresting Putin would amount to a declaration of war against Russia.