Ukrainian forces holding out in the ruins of Severodonetsk came under renewed heavy assault on Wednesday from Russian forces who see the capture of the industrial city as key to their control of the surrounding Luhansk region as President Volodymyr Zelenskyy acknowledged that the situation was difficult while pledging to retake Russia's gains.

The dayslong battle for the industrial city has emerged as pivotal, with Russia focusing its offensive might in the hope of achieving one of its stated aims – to fully capture the surrounding Luhansk province on behalf of Russian-speaking separatists.

Ukraine's military may have to pull back to stronger positions in the embattled eastern city of Severodonetsk, but they will not give up the city and fierce fighting raged there on Wednesday, the Luhansk region's governor said.

"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.

"We expect the amount of shelling and bombardments of Lyshychansk and Severodonetsk to increase many times, huge offensives in the Severodonetsk and Popasna direction and attempts to once again cross the Severskyi Donets River to create a bridgehead and further develop the offensive," Gaidai said.

He said Russia's key objective in the coming days is to capture Severodonetsk and to completely cut the strategically important road from the cities of Bakhmut and Lysychansk.

"The battles will be very fierce and they will give up everything they have to accomplish this task," he said.

"We have to achieve a full deoccupation of our entire territory," Zelenskyy said by video link at an event hosted by Britain's Financial Times newspaper on Tuesday.

Asked about comments by France's Emmanuel Macron that it was important not to "humiliate" Moscow, interpreted in Ukraine as implying some demands must be accepted, Zelenskyy said: "We are not going to humiliate anyone, we are going to respond in kind."

Severodonetsk, Lysychansk and Popasna remain the most difficult places, Zelenskyy said late on Tuesday.

Moscow said its troops have been advancing, while Zelenskyy said the "heroic defense" of Donbass was ongoing. Reuters could not independently verify the situation on the ground.

Since being pushed back from Kyiv and Kharkiv, Russia has focused on the region known as the Donbass, comprised of Luhansk and Donetsk provinces, and closest to the Russian border.

Russia drew closer to its goal of fully capturing Ukraine’s eastern industrial heartland of coal mines and factories as the Kremlin claimed to have taken control of 97% of one of the two provinces that make up the Donbass region.

Russian Defense Minister Sergei Shoigu said Tuesday that Moscow’s forces hold nearly all of Luhansk province. And it appears that Russia now occupies roughly half of Donetsk province, according to Ukrainian officials and military analysts.

Moscow says it is engaged in a "special military operation" to disarm and "denazify" its neighbor.

Ukraine and allies call this a baseless pretext for a war that has killed thousands, flattened cities and forced millions of people to flee.

'Book of executioners'

Zelenskyy 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 that its forces have committed war crimes.

The conflict is having a massive impact on the world economy. 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 worldwide 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.

The governor of the region that includes the port of Mykolaiv said weekend shelling had destroyed warehouses in one of Ukraine's largest agricultural commodities terminals.

Russian Defense 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 1,000 Ukrainian soldiers who surrendered there have been transferred to Russia for investigation.

Zelenskyy'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 Donbass after failing to take the capital Kyiv in March. Last month they were also driven back from the outskirts of Ukraine's second city, Kharkiv. The city, near the Russian border, was hit by fresh shelling on Tuesday.