President Volodymyr Zelenskiy said the raising of the Ukrainian flag on Snake Island in the Black Sea was a sign his country would not be broken, as President Vladimir Putin warned the West that its efforts to defeat him would bring tragedy to Ukraine.

In a hawkish speech to parliamentary leaders more than four months into the war, Putin said Russia had barely got started in Ukraine and the prospects for negotiation would grow dimmer the longer the conflict dragged on.

"We have heard many times that the West wants to fight us to the last Ukrainian. This is a tragedy for the Ukrainian people, but it seems that everything is heading towards this," he said.

Zelenskiy, in his nightly video message on Thursday, responded with defiance, saying that the two-month operation to retake Snake Island was a warning to all Russian forces.

"Let every Russian captain, aboard a ship or a plane, see the Ukrainian flag on Snake Island and let him know that our country will not be broken," he said.

Snake Island, a speck south of the port of Odesa, has become a symbol of Ukrainian determination.

In February, when ordered to surrender the small Ukrainian garrison on the island swore at their Russian attackers and were hit by an air strike.

Russia abandoned the island at the end of June in what it said was a gesture of goodwill - a victory for Ukraine that Kyiv hoped could loosen Moscow's blockade of Ukrainian ports.

On Thursday Ukraine's raised its blue-and-yellow flag on its recaptured Snake Island. Moscow responded with its warplanes striking the island and destroying part of the Ukrainian detachment there, it said.

Putin invaded Ukraine on Feb. 24, in what he called a "special military operation" to demilitarise Ukraine, root out dangerous nationalists and protect Russian speakers. Ukraine and its allies say Russia launched an imperial-style land grab.

The biggest conflict in Europe since World War Two has killed thousands, displaced millions and flattened scores of Ukrainian cities. Kyiv and the West accuse Russian forces of war crimes, but Moscow says it does not target civilians.

KEY ALLY LOST

On Thursday, Kyiv lost one of its main international supporters after British Prime Minister Boris Johnson said he would step down. Moscow did not conceal its delight at the political demise of a leader whom it has long criticised for arming Kyiv so energetically.

In a phone call, Johnson told Ukraine's President Volodymyr Zelenskiy "You're a hero, everybody loves you," a spokesman for Johnson said.

"Britain's support for Ukraine will not change whatever happens in the corridors of power in London. Boris and all our friends in the United Kingdom have assured us of that," Zelenskiy said in his nightly video address.

Johnson's resignation comes at a time of domestic turmoil in some other European countries that support Kyiv and doubts about their staying power for what has become a protracted conflict.

After failing to quickly take the capital Kyiv, Russia is now engaged in a war of attrition in Ukraine's eastern, industrial heartland of the Donbas.

On Sunday, Moscow declared it had "liberated" the Luhansk region and now plans to capture parts of neighbouring Donetsk it does not control. Luhansk and Donetsk make up the Donbas.

The mayor of the Donetsk city of Kramatorsk said Russian forces had fired missiles at the city centre in an air strike on Thursday and that at least one person was killed and six wounded.

Pavlo Kyrylenko, governor of the Donetsk region, said the missile had damaged six buildings including a hotel and an apartment bloc in the large industrial hub.

Reuters could not independently verify those assertions.

RUSSIAN 'FANTASY'

In Kramatorsk, mechanic-turned-soldier Artchk helped shore up defences against imminent Russian attack while, nearby, farmer Vasyl Avramenko lamented the loss of crops supplanted by mines.

"Of course we're already prepared. We're ready," Artchk, identifying himself by his nom-de-guerre, told Reuters.

"It's their (Russians') fantasy to occupy these cities, but they don't expect the level of resistance. It's not just the Ukrainian government, it's the people who refuse to accept them."

The regional governor of the northeastern city of Kharkiv said late on Thursday that three people had been killed and another five wounded after Russian forces shelled the city.

In the aftermath, bodies lying on the ground near a park bench were covered in sheets by emergency services. Two women who had gone out to feed cats in the area had been killed, said local resident Yurii Chernomorets.

A man fell to his knees weeping as the bloodied corpse of his wife was placed in a body bag. He kissed her hand.

"Dad, she is dead, please get up," said a man who identified himself as their son.

Reuters