In its most explicit threat yet, Russia’s military announced it would deem all ships heading for Ukrainian waters from Thursday morning to be potentially carrying weapons, and their flag countries as parties to the war on the Ukrainian side.

It said it was declaring parts of the Black Sea to be unsafe.

Washington called this a signal Moscow might attack civilian shipping and said Russia was also releasing new mines into the sea.

“We believe that this is a co-ordinated effort to justify any attacks against civilian ships in the Black Sea and lay blame on Ukraine for these attacks,” a White House National Security Council spokesman said.

The signals that Moscow was willing to use force to reimpose its blockade of one of the world’s biggest food exporters set global prices soaring.

Moscow says it will not participate in the year-old grain deal without better terms for its own food and fertiliser sales.

The UN says Russia’s decision threatens food security for the world’s poorest people.

Kyiv is hoping to resume exports without Russia’s participation but no ships have sailed from its ports since Moscow pulled out of the deal on Monday, and insurers have had doubts about whether to underwrite policies for trade in a war zone.

Since quitting the deal, Moscow has rained missiles down nightly on Ukraine’s two biggest port cities, Odesa and Mykolaiv.

Thursday’s strikes appeared to be the worst yet, with local authorities reporting at least 19 people wounded in Mykolaiv and eight wounded and one killed in Odesa.

The Russian strikes damaged a building at China’s consulate in Odesa, regional governor Oleh Kiper said.

He posted an image online of the building with broken windows.

The consulate is located in Odesa’s city centre just across railway tracks from the port.

“The aggressor is deliberately hitting the port infrastructure – administrative and residential buildings nearby were damaged, also the consulate of the People’s Republic of China,” Kiper said on Telegram.

“It shows the enemy does not pay attention to anything.”

Beijing made no immediate public mention of the incident, which took place while Ukraine’s Economy Minister Taras Kachka was in China for the first high-level Ukrainian visit since Russia’s invasion.

China said it told Kachka it was willing to expand imports of Ukrainian goods.

Kyiv has long sought to persuade Beijing to distance itself from Moscow.

In Mykolaiv, firefighters battled a huge blaze at a pink stucco residential building blasted into a ruin.

Several other residential buildings there were also damaged.

Moscow has described the port attacks as revenge for a Ukrainian strike on Russia’s bridge to Crimea on Monday.

It said on Thursday its retaliatory strikes were continuing and it had hit all its targets in Odesa and Mykolaiv.

The escalation in the Black Sea comes as Kyiv reports a new attempt by Russia to return to the offensive in the northeast of Ukraine, where it says Moscow has massed 100,000 troops and hundreds of tanks.

On the front line near Kupiansk, a railway hub Ukraine recaptured last month, Stanislav, an artillery unit commander, said his forces had received newly issued cluster munitions, and could start firing them soon – “maybe today or tomorrow”.

The US has started sending Ukraine cluster munitions although many countries have banned them as a potential danger to civilians.

Kyiv says the munitions, which contain scores of small bomblets that rain shrapnel over an area, could save lives if used in narrow circumstances to hasten the collapse of Russia’s front line.

Since last month, Ukrainian forces have been on the march in the east and the south, recapturing small amounts of territory in their first big counteroffensive since last year.

-with AAP