Ukrainian fighters were holding out on Thursday in the eastern city of Sievierodonetsk despite relentless Russian shelling, the regional governor said, as Moscow presses an assault that both sides believe could help shape the war's course.

Russia has concentrated its troops and firepower on the small industrial city to secure the surrounding province on behalf of Moscow-backed separatists. Ukraine's forces pulled back to the city's outskirts on Wednesday but have vowed to fight there for as long as possible.

Artillery shelling has turned the city in Ukraine's Luhansk province to a bombed-out wasteland. Luhansk's regional governor, Serhiy Gaidai, said the centre of the town was being destroyed.

"Our fighters are hanging on in the Sievierodonetsk industrial zone. But fighting is going on not just in the industrial zone, but right in the city of Sievierodonetsk," Gaidai told Ukrainian television late on Wednesday.

Gaidai said Russia now controlled more than 98% of Luhansk, claimed by Moscow for its proxies who have held eastern parts of the region since 2014.

Moscow has been trying to encircle Ukrainian forces in the areas they still hold. Ukrainian forces still control all of Sievierodonetsk' smaller twin city Lysychansk on the west bank of the Siverskyi Donets River but Russian forces were destroying residential buildings there, Gaidai said.

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

Russia has turned its focus to Luhansk and the adjacent province Donetsk since its forces were pushed back from the outskirts of the capital Kyiv in March.

Kyiv's ambassador to the United States told CNN that Ukrainian troops were vastly outnumbered in the fighting in Luhansk and Donetsk, which collectively form a region called the Donbas.

But "as we already saw in the battle for Kyiv, we can lose something temporarily. Of course, we're trying to minimize that because we know what (can) happen (when) Russians control territories, but we will get it back," Oksana Markarova said.

'GOD SAVED ME'

West of Sievierodonetsk in Sloviansk, one of the main Donbas cities in Ukrainian hands, women with small children lined up to collect aid on Wednesday while other residents carried buckets of water across the city.

Most residents have fled but authorities say around 24,000 remain in the city, in the path of an expected assault by Russian forces regrouping to the north.

Albina Petrovna, 85, described the moment her building was caught in an attack, which left her windows shattered and her balcony destroyed.

"Broken glass fell on me but God saved me, I have scratches everywhere...," she said.

Ukraine's military said four people were killed during Russian shelling on around 20 towns in the Donbas over the past 24 hours, and that its troops had killed 31 Russian soldiers. Reuters could not immediately verify the figures.

Moscow says it is engaged in a "special military operation" to disarm and "denazify" its neighbour. Ukraine and its allies say Moscow has launched an unprovoked war of aggression, killing thousands of civilians and flattening cities.

United Nations figures show more than 7 million people have crossed the border from Ukraine since Russia invaded on Feb. 24.

GRAIN SCARE

Ukraine is one of the world's biggest grain exporters, and Western countries accuse Russia of creating a risk of global famine by blockading Ukraine's Black Sea and Azov Sea ports. Moscow says Western sanctions are responsible for food shortages.

Turkey has been trying to broker negotiations to open up Ukraine's Black Sea ports. Foreign Minister Mevlut Cavusoglu hosted Russian Foreign Minister Sergei Lavrov on Wednesday and said a U.N.-backed deal on the ports was possible with further talks.

Lavrov said the Ukrainian ports could be opened, but Ukraine would have to de-mine them first. Ukraine dismissed Russia's assurances as "empty words" and said Russian attacks on farmland and agricultural sites were exacerbating the crisis.

Vitaliy Kim, governor of the Mykolaiv region, where Russian shelling destroyed the warehouses of one of Ukraine's largest agricultural commodities terminals over the weekend, told Reuters that Moscow was trying to scare the world into meeting its terms.

The Kremlin cited Russian President Vladimir Putin as saying Western sanctions must be lifted for Russian grain to reach markets.

Ukraine's president, Volodymyr Zelenskiy, told a Yale University summit of business leaders by video link on Wednesday that he believes Russia will not seek a diplomatic end to the war unless the world supports Ukrainian troops in their fight.

"We are an independent, righteous, normal country," Zelenskiy said, adding about his troops' war efforts: "We do it on our land and we slowly push them back. That's how we're going to keep on moving."