Heavy Russian strikes hit the southern Ukrainian port city of Mykolaiv overnight and early on Sunday, killing the owner of one of the country’s largest grain producing and exporting companies, the local governor said.

Oleksiy Vadatursky, founder and owner of agriculture company Nibulon and his wife, were killed in their home, Mykolaiv governor Vitaliy Kim said on Telegram.

Headquartered in Mykolaiv, a strategically important city that borders the Russia-occupied Kherson region, Nibulon specialises in the production and export of wheat, barley and corn, and it has its own fleet and shipyard.

President Volodymyr Zelensky described Vadatursky’s death as “a great loss for all of Ukraine”, saying in a statement the businessman had been in the process of building a modern grain market involving a network of transhipment terminals and elevators.

Three people were also wounded in the attacks on Mikolaiv, the city’s mayor, Oleksandr Senkevych, told Ukrainian television, adding 12 missiles had hit homes and educational facilities.

Up to 50 Grad rockets hit residential areas in another southern city, Nikopol, on Sunday morning, Dnipropetrovsk governor Valentyn Reznichenko wrote on Telegram. One person was wounded.

Ukrainian forces struck the headquarters of Russia’s Black Sea Fleet in Russian-held Sevastopol early on Sunday, the Crimean port city governor, Mikhail Razvozhayev told Russian media. Five members of staff were wounded in the attack when what was presumed to be a drone flew into the courtyard at the headquarters, he said.

The Sevastopol attack coincided with Russia’s Navy Day, which President Vladimir Putin marked by announcing that the Russian navy would receive what he called “formidable” hypersonic Zircon cruise missiles in coming months. Those missiles can travel at nine times the speed of sound.

He did not mention Ukraine directly.

Zelensky said on Sunday the country may harvest only half its usual amount this year due to the invasion.

“Ukrainian harvest this year is under the threat to be twice less,” suggesting half as much as usual, Zelensky wrote in English on Twitter. “Our main goal — to prevent global food crisis caused by Russian invasion. Still grains find a way to be delivered alternatively,” he added.

There is a high possibility that the first grain-exporting ship will leave Ukraine’s ports on Monday, a spokesperson for Turkish President Recep Tayyip Erdogan has said.

In a televised address late on Saturday, Zelensky said hundreds of thousands of people were still exposed to fierce fighting in the Donbas region, which contains Donetsk and Luhansk provinces and which Russia seeks to control. Swathes of the Donbas were held before the invasion by Russian-backed separatists.

“Many refuse to leave but it still needs to be done,” Zelensky said. “The more people leave the Donetsk region now, the fewer people the Russian army will have time to kill.”

At the Navy Day event in St Petersburg, Putin signed a naval doctrine that cast the US as Russia’s main rival and set out Russia’s global maritime ambitions for crucial areas such as the Arctic and in the Black Sea.

The 55-page naval doctrine sets out the broad strategic aims of Russia’s navy, including its ambitions as a “great maritime power” which extend over the entire world.

The main threat to Russia, the doctrine says, is “the strategic policy of the USA to dominate the world’s oceans” and the movement of the Nato military alliance closer towards Russia’s borders.

“The Russian Federation will firmly and resolutely defend its national interests in the world’s oceans, and having sufficient maritime power will guarantee their security and protection,” the document said.