Russian forces left a trail of destruction in their wake Thursday as they grinded deeper into Donbas with their sights set on the industrial hub of Sloviansk in eastern Ukraine as their next target.

Moscow's slow push came as diplomatic tensions mounted between Ankara and Kyiv, where Ukrainian officials accused Turkey of ignoring calls to seize grain being transported by a Russian ship.

Russian forces killed at least seven civilians and injured others in the last 24 hours throughout the battle-scarred Donetsk region, the region's head Pavlo Kyrylenko said Thursday, in Moscow's latest wave of deadly attacks.

Kramatorsk, Ukraine's de-facto administrative centre in Donetsk was struck by Russia on Thursday, AFP journalists said, killing at least one civilian and injuring several others.

The explosion left a large crater in a courtyard between a hotel and residential buildings and several cars were on the fire.

The fatalities came after Ukrainian officials re-issued urgent pleas for civilians in the war-torn region to flee as Russian forces turn on Sloviansk.

Vitaliy, a Ukrainian plumber, told AFP in the industrial hub that his wife and her daughter from a previous marriage, who is six months pregnant, were evacuated from the city the day before.

"I sent my wife (away), and I have no more choice: tomorrow I will join the army."

The town has been hit repeatedly by Russian bombardments and on Wednesday a marketplace and its surrounding streets were badly damaged in a barrage of rockets.

Well fortified

Despite the threat of intensifying Russian bombardments as Moscow turns its military focus on the city, some residents vowed to stay.

"We have basements, we will hide there," said 72-year-old greengrocer Galyna Vasyliivna.

"What we can do? We have nowhere to go, nobody needs us."

Mayor Vadym Lyakh said around 23,000 people remained out of a pre-war population of 110,000 and claimed Russia had been unable to surround the city.

"Evacuation is ongoing. We take people out every day," he said, explaining that fleeing civilians are being brought by bus to the city of Dnipro, further west.

"The city is well fortified," he said.

Sloviansk in the Donetsk region is next in Russia's line of fire after their capture of the nearby sister cities of Severodonetsk and Lysychansk in the Lugansk region.

The fall of Lysychansk last week after the Ukrainian army also retreated from Severodonetsk has freed up Russian troops, who switched their attention to the Donbas after being beaten back from around the capital Kyiv and Ukraine's second city Kharkiv early in the invasion.

Otherwise, a diplomatic crisis flared between Ukraine and Turkey over the apparent transport by Russia of grain allegedly stolen from Ukraine.

Deeply disappointed

Kyiv alleges that a 7,000-tonne vessel, the Zhibek Zholy, set off from Ukraine's Kremlin-occupied port of Berdyansk after picking up confiscated wheat and called last week for Turkey to seize it.

The marinetraffic.com website showed on Thursday the vessel moving away from Turkey's Black Sea port of Karasu before apparently switching off its transponder and disappearing from view.

Ukraine said it was "deeply disappointed" that Turkey had not acted on its request to seize the ship.

The incident points to Turkey's complicated role in the war, where President Recep Tayyip Erdogan has a close but tumultuous relationship with Moscow but also supplies combat drones to Ukraine.

Despite Ukraine's recent territorial losses, President Volodymyr Zelensky in an evening address on Wednesday praised new heavy Western artillery for boosting Ukraine's firepower.

"The weapons we have received from our partners have started working very powerfully. Their accuracy is exactly as it should be," he said.

"Our defenders inflict notable blows on warehouses and other points which are important for the logistics of the occupiers," he said.

The EU meanwhile set out a harder focus on energy given the war with European Commission chief Ursula von der Leyen warning of the need to "prepare for further disruptions of gas supply, even a complete cut-off from Russia".

The European Union has launched a 300-billion-euro ($310-billion) plan to wean itself off Russian fossil fuel supplies.