Moscow's forces also struck the Black Sea port of Odesa for a fourth night in a row, hitting grain silos, officials said.

After pulling out of a deal facilitating the safe shipment of grain from Ukraine, Russia has been targeting the Western-backed country's grain supplies and vital infrastructure in its southern ports including Odesa and Mykolaiv.

"The Russians attacked Odesa with Kalibr cruise missiles from the Black Sea," said regional governor Oleg Kiper.

Moscow targeted local grain silos and "destroyed 100 tons of peas and 20 tons of barley," Kiper said, adding two people were injured.

United Nations Secretary-General Antonio Guterres said the effect of such attacks went well beyond Ukraine.

"We are already seeing the negative effect on global wheat and corn prices which hurts everyone, but especially vulnerable people in the global south," Guterres said in a statement.

Russian President Vladimir Putin this week vowed to exact revenge after a Ukrainian attack damaged a bridge linking annexed Crimea to Russia and killed two people.

In eastern Ukraine, a couple was killed in Russian shelling of the city of Kostiantynivka, Donetsk regional governor Pavlo Kyrylenko said.

In Moscow, the defence ministry said that a missile boat "carried out live firing of anti-ship cruise missiles at the target ship" in the northwestern part of the Black Sea.

"The target ship was destroyed as a result of a missile strike," the ministry said in a statement.

Ships and fleet aviation had also "worked out actions to isolate the area temporarily closed to navigation, and also carried out a set of measures to detain the offending ship," the ministry added.

Russia and China's navies have also carried out a joint exercise in the Sea of Japan.

The Kremlin said Wednesday it would consider cargo ships destined for Ukraine via the Black Sea potential military targets.

Ukraine has also warned that from Friday it may consider vessels heading to Russian ports "as carrying military cargo, with all the associated risks".

Ukraine has previously said it would be ready to continue with grain exports from its southern ports following Moscow's exit from the deal.

Kyiv has called on the UN and neighbouring countries to secure safe passage for cargoes through joint patrols.

- Zelensky sacks ambassador -

In Kyiv, Ukrainian leader Volodymyr Zelensky dismissed the country's ambassador to the United Kingdom, Vadym Prystaiko, after he criticised the president's response to a row over British military aid.

Prystaiko's dismissal came after he criticised Zelensky's sarcastic response to suggestions from British defence minister Ben Wallace that Ukraine should show more gratitude for arms supplies from its allies.

The row began when Wallace told journalists at a NATO summit in Vilnius this month that Britain is not an Amazon delivery service for weapons to Ukraine and suggested Kyiv could express more "gratitude".

Zelensky responded at a press conference, saying he did not know how else to make clear Ukraine's gratitude. "We could wake up in the morning and express our words of gratitude to the minister personally," he said.

On the battlefield, Ukrainian forces have begun using United States-supplied cluster munitions, the White House said, as Kyiv seeks momentum in its grinding counteroffensive.

Washington provided the weapons to Ukraine for the first time earlier this month as Kyiv attempts to dislodge entrenched Russian forces and retake land lost in the early months of Moscow's military operation last year.

The weapons, which disperse up to several hundred small explosive charges that can remain unexploded on the ground, are banned by many countries because of the long-term risks they pose to civilians.

Ukraine's forces started using the munitions "in the last week or so", White House National Security Council spokesman John Kirby said on Thursday.

"They're using them appropriately, they're using them effectively and they are actually having an impact on Russia's defensive formations and Russia's defensive manoeuvring," he said.

Moscow's forces are entrenched across swathes of southern and eastern Ukraine and over a month into Kyiv's counteroffensive, large parts of the front appear to be frozen.

This week a senior presidential aide in Kyiv told AFP the operation would be "long and difficult".

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