Russian forces have advanced deep into the ruined eastern factory city of Sievierodonetsk, but Ukrainian troops are still holding out there as Russia’s assault on its neighbour entered its 100th day.

A war that Western countries believe Russia planned to win within hours has ground on for more than three months, with Moscow having been driven back from the capital Kyiv but launching a huge new assault in the east.

The past weeks have seen Russia pour its forces into the battle for Sievierodonetsk, a small factory city in the east, which Russia must capture to achieve its stated aim of holding all of Luhansk province.

Both sides have been taking punishing losses there in a street-by-street battle that could set the trajectory for a long war of attrition to come.

Ukrainian regional governor Serhiy Gaidai said about a fifth of the city was now a contested “grey zone”.

“I regret to say that the Russian army succeeded in making its way deep into the city … they control most of the city,” he said in televised comments overnight.

Deadliest phase so far

He said Ukrainian fighters were holding out in one area, were still able to clear Russians out of some streets, and had captured six Russian prisoners the previous day.

“So I would tell sceptics not to write off Sievierodonetsk. It’s too early to do that. The city is holding on.”

Despite being driven from the north of Ukraine in March after a failed assault on the capital, Russia still controls around a fifth of Ukraine’s territory, about half of which it seized in 2014 and half it captured since launching its invasion on February 24.

The massive Russian assault in the east in recent weeks has been one of the deadliest phases of the war for both sides.

Moscow has made slow but steady progress, squeezing Ukrainian forces inside a pocket in Luhansk and Donetsk provinces, but has so far failed to encircle them.

Ukrainian troops are being forced back in Sievierodonetsk, but still firmly hold its twin city Lysychansk across the Siverskyi Donets river.

Kyiv, meanwhile, is hoping that the Russian advance will leave Moscow’s forces so depleted that Ukraine will be able to launch counter-offensives and recapture territory in the months to come.

Replenishing the arsenal

It has been bolstered in recent days by a promise of medium-range missiles from the United States that would let it strike deep behind Russian lines and neutralise Moscow’s firepower advantage.

Ukrainian President Volodymyr Zelenskiy is expecting more good news on weapon supplies from other partners.

“We are working to bring the supply of modern combat systems to a much higher level,” Zelenskiy said in an overnight address.

“The entire temporarily occupied territory of our state is now a complete disaster zone, for which Russia bears full responsibility,” he said.

Russia has accused the United States of adding “fuel to the fire” with a new $US700 million ($A964 million) weapons package for Ukraine that will include advanced rocket systems with a range of up to 80km.

US President Joe Biden’s administration has repeatedly said it had Ukraine’s assurances it would not use the rocket systems to hit targets inside Russia.

-AAP