Russia jolted world grain markets with an escalation in the Black Sea, mounting a third straight night of air strikes on Ukrainian ports and issuing a threat against Ukraine-bound vessels to which Kyiv responded in kind.

At least 27 civilians were reported hurt in the air strikes on the ports, which set buildings ablaze and damaged China's consulate in Odesa.

The US said Russia's warning to ships indicated Moscow might attack vessels at sea following Moscow's withdrawal from a UN-brokered deal to let Ukraine export grain. The signals that Russia 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.

Odesa regional governor Oleh Kiper posted an image online of China's consulate building with broken windows. It 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. It shows the enemy does not pay attention to anything," Kiper said on Telegram.

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

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.

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.

Kyiv responded on Thursday by saying it would consider vessels bound for Russia also to be carrying arms.