Kyiv said on Saturday it was pushing back Russian troops in Sievierodonetsk as intense fighting raged around the industrial city, the focus of a Russian offensive to take the eastern Donbas region, news agency Reuters reported.
Serhiy Gaidai, governor of Luhansk province, said Russian forces had suffered severe defeats and were blowing up bridges across the Siverskyi Donets river to prevent Ukraine bringing in military reinforcements and delivering aid to civilians in Sievierodonetsk.
"Right now, our soldiers have pushed them back, they (the Russians) are suffering huge casualties," Gaidai said in a live TV broadcast on Saturday.
"The Russian army, as we understand, is throwing all its efforts, all its reserves in that (Sievierodonetsk) direction," he said. "Russians are blowing up bridges, so we could not bring in reinforcements to our boys in Sievierodonetsk."
The governor said Ukrainian forces had recaptured around one-fifth of the territory they had lost in the city.
Both sides have suffered punishing losses in street-by-street battles for the Soviet-era factory city, where roads have been riddled with craters and destroyed vehicles.
If Sievierodonetsk falls, neighbouring Lysychansk would be the last city that Russia needs to capture to have full control of Luhansk province, which along with Donetsk province makes up the Donbas.
The area has become the focus of Russia's invasion as President Vladimir Putin seeks to rebuild momentum after a failed attempt to take the capital Kyiv.
Ukraine's military said on Saturday Russia had used artillery to conduct "assault operations" in Sievierodonetsk, but Russian forces retreated and Ukrainian troops are holding positions inside the city.
Reuters reached Sievierodonetsk on Thursday and was able to verify that Ukrainians still held part of the city.
Russian soldiers also attempted to advance towards Lysychansk, across the Siverskyi Donets river from Sievierodonetsk, but were stopped, Ukraine's military general staff said.
Britain's defence ministry said on Saturday that Russian air activity remains high over Donbas, with Russian aircraft carrying out strikes using both guided and unguided munitions.
Russia's defence ministry said on Saturday its forces shot down a Ukrainian military transport plane carrying weapons and munitions near the Black Sea port of Odesa.
GRAIN SUPPLIES CUT OFF
Tens of thousands have died, millions have been uprooted from their homes and the global economy disrupted in a war that marked its 100th day on Friday.
French President Emmanuel Macron, who has sought to maintain a dialogue with Putin throughout the war, said on Saturday it is vital that Russia is not humiliated so that when the fighting stops a diplomatic solution can be found.
Macron's stance has been repeatedly criticised by some eastern and Baltic partners in Europe.
Ukraine said on Saturday there was no point in negotiating with Russia until Moscow's forces are pushed back as far as possible towards Ukraine's borders.
Ukrainian officials are counting on the advanced missile systems recently pledged by the United States and Britain to swing the war in their favour, and Ukrainian troops have already begun training on them.
Moscow has said the Western weapons will pour "fuel on the fire," but will not change the course of what it calls a "special military operation" to disarm Ukraine and rid it of dangerous nationalists.
The war has had a devastating impact on the global economy, especially for poor food-importing countries.
Ukraine is one of the world's leading sources of grain and cooking oil, but those supplies were largely cut off by Russia's closure of its Black Sea ports, with more than 20 million tonnes of grain stuck in silos.
Kyiv and its allies blame Moscow for blockading the ports.
Putin denied on Friday that Moscow was preventing Ukrainian ports from exporting grain, blaming rising global food prices on the West.
Moscow says sanctions are also hitting its own grain and fertiliser exports, aggravating the shortages.
UN Secretary-General Antonio Guterres has this week tried to broker what he calls a "package deal" to resume both Ukrainian food exports and Russian food and fertiliser exports.