Ukraine said on Thursday its forces were holding their positions in intense fighting in the eastern city of Sievierodonetsk and had retaken ground in the south, targetting gains in the biggest swathe of territory seized by Russia since the invasion started.

Russia has concentrated its invasion force around Sievierodonetsk, a small industry city now bombed to ruins. Ukraine says its only hope to turn the tide toward victory is more artillery to offset Russia's massive firepower.

Sievierodonetsk and its twin city Lysychansk on the opposite bank of the Siverskyi Donets river are the last Ukrainian-controlled parts of Luhansk province, which Moscow is determined to seize as one of its principal war objectives.

"They (the Russians) are dying like flies ... fierce fighting continues inside Sievierodonetsk," Luhansk governor Serhiy Gaidai said.

Gaidai predicted the Russians would try to take advantage of low water levels to cross the Siverskyi Donets river. "We are watching and if anything happens we will act proactively."

In the south, where Moscow is trying to impose its rule on a tract of occupied territory spanning Kherson and Zaporizhzhia provinces, Ukraine's defence ministry said it had captured new ground in a counter-attack in Kherson province.

President Volodymyr Zelenskiy said in an evening address that Ukraine also had "some positive developments in the Zaporizhzhia region, where we are succeeding in disrupting the occupiers' plans." He did not provide details.

Reuters could not independently verify the situation on the ground in Zaporizhzhia or Kherson. Russian-installed proxies in both provinces say they are planning referendums to join Russia.

Thousands of people have been killed and millions have fled since Moscow launched its "special military operation" to disarm and "denazify" its neighbour on Feb 24. Ukraine and its allies call the invasion an unprovoked war of aggression.

'We are starving'

Speaking in Moscow to mark the 350th anniversary of Russian Tsar Peter the Great's birth, President Vladimir Putin drew a parallel between what he portrayed as their historic quests to win back what he called Russian lands.

"Peter the Great waged the Great Northern War for 21 years. It would seem that he was at war with Sweden, he took something from them. He did not take anything from them, he returned (what was Russia's)," Putin said.

In a rare update from Sievierodonetsk, the commander of Ukraine's Svoboda National Guard Battalion, Petro Kusyk, said Ukrainians were drawing the Russians into street fighting to neutralise Russia's artillery advantage.

"Yesterday was successful for us - we launched a counteroffensive and in some areas we managed to push them back one or two blocks. In others they pushed us back, but just by a building or two," he said in a televised interview.

But he added his forces were suffering from a "catastrophic" lack of counter-battery artillery to fire back at Russia's guns, and getting such weapons would transform the battlefield.

Sievierodonetsk Mayor Oleksandr Stryuk said around 10,000 civilians were still trapped inside the city - around a tenth of its pre-war population.

Reuters could not verify the situation on the ground in the city.

To the west of Sievierodonetsk, Russia is pushing from the north and south, trying to trap Ukrainian forces in the Donbas region comprising Luhansk and neighbouring Donetsk province.

In Soledar, a salt-mining town near Bakhmut close to the front line, buildings had been blasted into craters.

Remaining residents, mostly elderly, were sheltering in a crowded cellar. Antonina, 65, had ventured out to see her garden. "We are staying. We live here. We were born here," she sobbed. "When is it all going to end?"

Grain

In the self-proclaimed Donetsk People's Republic (DPR), one of Russia's proxies in eastern Ukraine, a court sentenced two Britons and a Moroccan who were captured while fighting for Ukraine to death, Russian news agencies reported.

Britain slammed the court's decision as a "sham judgment" with no legitimacy.

Ukraine is one of the world's biggest grain and food oil exporters, and international attention has focused in recent weeks on the threat of international famine seen as caused by Russia's blockade of Ukraine's Black Sea ports.

"Millions of people may starve if the Russian blockade of the Black Sea continues," Zelenskiy said on Thursday in televised remarks.

Moscow blames the food crisis on Western sanctions restricting its own grain exports. It says it is willing to let Ukrainian ports open for exports if Ukraine removes mines and meets other conditions. Kyiv calls such offers empty promises.

Turkey, a Nato power with good relations with both Kyiv and Moscow, has tried to mediate.

Russia has also been trying to sell grain from areas of Ukraine it seized, activity Kyiv and the West call looting.

Asked if any deal had been reached to sell grain from southern Ukraine to Turkey or a Middle Eastern country, Kremlin spokesperson Dmitry Peskov said: "So far no agreements have been reached, work is continuing."

- Reuters