Mykolaiv lies roughly 35 km northwest of the front line to occupied Kherson, the southern region where Russia has ordered 60,000 people “to save your lives” and flee a Ukrainian counter offensive.

Russian Defence Minister Sergei Shoigu, who some Russian nationalists have blamed for Moscow’s setbacks since the February 24 invasion, discussed the “rapidly deteriorating situation” in calls with French and Turkish counterparts, the ministry said.

Without providing evidence, Shoigu said Ukraine could escalate with a “dirty bomb” – conventional explosives laced with radioactive material. Ukraine does not possess nuclear weapons, while Russia has said it could protect Russian territory with its nuclear arsenal.

A Russian missile strike on Sunday wiped out the top floor of an apartment block in Mykolaiv, sending shrapnel and debris across a plaza and into neighbouring buildings, smashing windows and cracking walls. Cars were crushed under rubble, Reuters witnessed. No fatalities were recorded.

“After the first blast, I tried to get out, but the door was stuck. After a minute or two, there was a second loud blast. Our door was blown into the corridor,” said Oleksandr Mezinov, 50, who was awoken from his bed by the blasts.

Ukraine shot down 14 Russian “kamikaze” drones over Mykolaiv overnight, regional governor Vitaliy Kim said on Telegram. The drones are designed to explode on impact and have hammered Ukraine’s energy infrastructure this month.

Kim said Russia also attacked with S-300 missiles, one of which hit the five-storey apartment building.

Russian troops have withdrawn from parts of the front in recent weeks and occupation authorities are evacuating civilians deeper into Russian-held territory before an expected battle for Kherson, the regional capital on the west bank of the Dnipro river. Kherson is a gateway to Crimea, which Russia annexed in 2014.

“The situation today is difficult. It’s vital to save your lives,” Russian Education Minister Sergei Kravtsov said in a video message. “It won’t be for long. You will definitely return,” he added.

One man was killed and three injured after a blast in the city, a Russian state news agency said. Emergency services said an improvised explosive device was detonated near a car in the city.

Russia-installed authorities there reported a shortage of vessels to ferry people across the river at one point on Sunday, blaming a “sharp increase in the number of people wishing to leave.”

Around 25,000 people have been evacuated since Tuesday, the Interfax news agency said.

Ukraine’s military said it was making gains in the south, taking over at least two villages it said Russia had abandoned.

Reuters could not independently verify the accounts.

Ukraine’s advances in recent weeks around Kherson and in the country’s northeast have been met with intensifying Russian missile and drone attacks on civilian infrastructure, which have destroyed about 40 per cent of Ukraine’s power system ahead of winter.

Russia and Ukraine have accused each other of planning to blow up the Nova Kakhovka dam, which holds roughly as much water as the Great Salt Lake in the US state of Utah. Breaching it could flood a swathe of southern Ukraine, including Kherson.

Neither side has produced evidence to back up their claims about the dam, which supplies water to Crimea and the Russian-held Zaporizhzhia nuclear power plant.

In another setback for Moscow, a Russian military jet crashed into a residential building in the Siberian city of Irkutsk in Russia’s far east on Sunday, killing the two pilots, the second fatal incident in six days involving a Sukhoi fighter plane.

-AAP