Ukrainian troops holding out in the ruins of Sievierodonetsk have come under renewed heavy assault from Russian forces who see the capture of the industrial city as key to control of the surrounding Luhansk region.
In southern Ukraine, another major battleground in the war, authorities warned that Russian attacks on agricultural sites including warehouses were compounding a global food crisis that has stirred concerns of famine in some developing countries.
Russian forces have been focused for weeks now on seizing Sievierodonetsk, which was home to some 106,000 people before Moscow invaded Ukraine on February 24, but the Luhansk region's governor said Ukrainian forces would not surrender the city.
"Fighting is still raging and no one is going to give up the city, even if our military has to step back to stronger positions. This will not mean someone is giving up the city - no one will give up anything. But (they) may be forced to pull back," Serhiy Gaidai told Ukrainian television.
Russian forces will further increase their shelling and bombardment of both Sievierodonetsk and its smaller twin city of Lysychansk on the west bank of the Siverskyi Donets River, he said.
Luhansk and the adjacent province of Donetsk form the Donbas, claimed by Moscow for Russian-speaking separatists who have held eastern parts of the region since 2014.
"The absolutely heroic defence of Donbas is ongoing," Ukrainian President Volodymyr Zelenskiy said in a video statement on Tuesday.
"It is absolutely felt that the occupiers didn't believe the resistance of our military would be so strong and now they are trying to bring in new resources towards the Donbas," he said.
Zelenskiy said Ukraine would launch next week a "Book of Executioners" to detail war crimes.
"These are concrete facts about concrete individuals guilty of concrete cruel crimes against Ukrainians," he said.
Russia denies targeting civilians in Ukraine and rejects accusations its forces have committed war crimes.
Ukraine is one of the world's biggest exporters of grain, and Western countries accuse Russia of creating a risk of global famine by shutting Ukraine's Black Sea ports. Moscow denies blame and says Western sanctions are responsible for food shortages.
Russian attacks on agricultural sites in the Mykolaiv region were compounding a world-wide food crisis, Ukraine's southern military command said on Facebook on Wednesday.
"Those who pretend to be concerned about the world food crisis are in fact attacking farmland and infrastructure sites where fires of considerable scale have broken out," Ukraine's southern military command said on Facebook on Wednesday.
Russian Defence Minister Sergei Shoigu said the Russian-occupied Ukrainian ports of Berdyansk and Mariupol were ready to resume grain exports. Ukraine says any such shipments from territory seized by Moscow would amount to illegal looting.
More than two weeks since a siege of the southern city of Mariupol ended, TASS news agency cited a Russian law enforcement source as saying that over 1000 Ukrainian soldiers who surrendered there have been transferred to Russia for investigation.
Zelenskiy's office said two people were killed and two wounded in the Luhansk region in the past 24 hours, five civilians were wounded in the Donetsk region, and four killed and 11 wounded in the Kharkiv region.
Russian forces turned their focus to the Donbas after failing to take the capital Kyiv in March.
Ukraine's second city, Kharkiv, near the Russian border, was hit by fresh shelling on Tuesday.