Kraine was preparing his defenses on Wednesday for a massive Russian attack on key cities in eastern Donetsk province.
The country’s army would fight desperately to resist a predicted major offensive on Sloviansk and Kramatorsk.
It came days after Vladimir Putin’s forces claimed control of Ukraine’s last remaining stronghold in neighboring Luhansk, which together with Donetsk forms the Donbas region that Russia plans to take over from Kiev.
Luhansk’s governor said on Wednesday that there had been heavy fighting on the outskirts of the region.
Serhiy Gaidai told Ukrainian TV that the Russian regular army and reserve troops had been sent there in an apparent attempt to cross the Siversky Donets River.
“We are holding back the enemy on the border of the Luhansk region and the Donetsk region,” Gaidai wrote on Telegram.
Mr Gaidai previously said Russian troops were engaged in widespread shelling when they launched their attack on Donetsk.
On behalf of Moscow-backed separatists in two self-declared people’s republics, Russia says it wants to take control of the entire Donbas from Ukraine.
Russian forces struck a market and residential area in Sloviansk yesterday, killing at least two people and injuring seven, local officials said.
Donetsk regional governor Pavlo Kyrylenko said Sloviansk and nearby Kramatorsk had been subject to heavy shelling overnight. “There is no safe place without shelling in the Donetsk region.”
British defense chiefs said on Wednesday that Putin’s forces have advanced just five miles in a week on a key front as a result of “extremely determined Ukrainian resistance”.
His troops were estimated to be about ten miles from Sloviansk. It said there is a “realistic possibility that the battle for Sloviansk will be the next important match in the battle for the Donbas”.
Kiev has said it is trying to buy time to take fortified positions ahead of an expected Russian attack on Donetsk.
In his last nightly video address, Ukrainian President Volodymyr Zelenskyy said air-raid sirens have been issued almost all over the country. The warnings followed a period of relative calm for much of the country after Putin quickly abandoned his original plan to topple Kiev after launching his invasion on February 24.
“You shouldn’t look for logic in the actions of terrorists,” Zelensky said.
“The Russian army does not take breaks. It has one job – to take people’s lives, intimidate people – so that even a few days without an air raid siren feels part of the terror.”