Ukraine said the "strongest" shelling by Moscow so far of the southern city Mykolaiv killed a grain tycoon Sunday, as Russia claimed an attack from a drone wounded six personnel at the headquarters of its Black Sea fleet in annexed Crimea. 

AFP journalists witnessed intense Russian bombardment of the eastern town of Bakhmut after President Volodymyr Zelensky called for civilians to leave the front line Donetsk region bearing the brunt of the Kremlin's offensive.

Russian authorities in the Crimean Black Sea peninsula -- seized by Moscow from Ukraine in 2014 -- said a small explosive device from a commercial drone, likely launched nearby, hit the navy command in Sevastopol. 

The local mayor blamed "Ukrainian nationalists" for the attack that forced the cancellation of festivities in the city marking Russia's annual holiday celebrating the navy.

But a spokesman for Ukraine's Odessa region military administration denied Kyiv -- whose nearest positions are some 200 kilometres (125 miles) away -- was responsible and called the incident "a sheer provocation".

"Our liberation of Crimea from the occupiers will be carried out in another way and much more effectively," spokesman Sergiy Bratchuk wrote on Telegram. 

Authorities in Ukraine's southern city of Mykolaiv said Sunday that widespread Russian bombardments overnight had left at least two civilians dead, as Moscow continued to pummel the sprawling front line.

"Mykolaiv was subjected to mass shelling today. Probably the strongest so far," the city's mayor Oleksandr Senkevych wrote on Telegram.  

The authorities said leading Ukrainian agricultural magnate Oleksiy Vadatursky, 74, and his wife Raisa were killed when a missile struck their house. 

Vadatursky, who was ranked Ukraine's 24th richest man with a fortune worth $430 million by Forbes, owned major grain exporter Nibulon and was previously decorated with the prestigious "Hero of Ukraine" award.

Mykolaiv -- which has been attacked frequently -- is the closest Ukrainian city to the southern front where Kyiv's forces are looking to launch a major counter-offensive to recapture territory lost after Russia's February invasion. 

- Zelensky urges Donetsk evacuation -
 

Strikes also pounded the northeastern regions of Kharkiv and Sumy, near the front line with the Russian forces. 

"Today a whole succession of explosions took place... a few buildings are reportedly damaged," Igor Terekhov the mayor of Ukraine's second city Kharkiv said. 

Sumy regional chief Dmytro Zhyvytsky said that some 50 strikes on Saturday evening had left one person dead and two wounded. 

The governor of the Donetsk region, where Moscow is focusing the brunt of its attacks, said three civilians were killed and eight wounded in shelling Saturday. 

AFP journalists on Sunday saw one wounded man collected by an ambulance after a ferocious bombardment of the town of Bakhmut. 

In an overnight address Zelensky warned that thousands of people, including children, were still in the battleground areas of the Donetsk region.

"There's already a governmental decision about obligatory evacuation from Donetsk," Zelensky said, underscoring authorities' calls to leave the besieged region in recent weeks.

"Leave, we will help," Zelensky said. "At this stage of the war, terror is the main weapon of Russia."

Official Ukrainian estimates put the number of civilians still living in the unoccupied area of Donetsk at between 200,000 and 220,000.

A mandatory evacuation notice posted Saturday evening said the coming winter made it a matter of urgency, particularly for the more than 50,000 children still in the region.

- Deadly jail strike -
 

The call came a day after a jail holding Ukrainian prisoners of war in Kremlin-controlled Olenivka was bombed, leaving scores dead, with Kyiv and Moscow trading blame. 

Russia's defence ministry said Sunday it had invited the International Committee of the Red Cross (ICRC) and the United Nations to visit the site "in the interests of an objective investigation". 

There was no confirmation from the two international bodies. 

The ICRC said Saturday that a request to send a team to the site had not been granted, and Ukraine said it was encouraging international experts to go to Olenivka.   

Russia's military said Saturday that 50 Ukrainian servicemen died, including troops who had surrendered after weeks of fighting off Russia's brutal bombardment of the sprawling Azovstal steelworks in the port city of Mariupol.

Ukraine says Russia was behind the attack, with Zelensky accusing the Moscow of the "deliberate mass murder of Ukrainian prisoners of war".

He has stepped up calls for the international community, especially the United States, to have Russia officially declared a "state sponsor of terrorism".

The intense bombardments around Ukraine come as the authorities push to restart grain exports under a plan brokered by the UN and Turkey to lift a Russian naval blockade. 

A spokesman for the Turkish presidency said there was a "high probability" that a first ship carrying Ukrainian grain could leave Ukraine on Monday.