Ukraine will formally become a candidate to join the European Union on Thursday, a boost for the devastated country's morale as Russian assaults wear down the defenders of two cities in the eastern Donbas region.

Although the approval of the Kyiv government's application by EU leaders meeting in Brussels is just the start of what will be a years-long process, it marks a huge geopolitical shift and will anger Russia as it struggles to impose its will on Ukraine.

The expected green light "is a signal to Moscow that Ukraine, and also other countries from the former Soviet Union, cannot belong to the Russian spheres of influence," Ukraine's ambassador to the EU, Vsevolod Chentsov, told Reuters.

In the capital, 22-year-old serviceman Volodymyr Yanishan welcomed the move to candidate status. "It means that people almost reached what we have been striving for since 2014 in a bloody fight which cost us much effort... I think the majority will be glad and it means changes for better."

Mass protests in Kyiv that year ousted Ukraine's then-president after he broke a promise to develop closer ties with the EU. 

Friday will mark four months since Russian President Vladimir Putin sent troops across the border in what he calls a "special military operation" partly necessitated by Western encroachment into what Russia considers its sphere of influence.

The conflict, which the West sees as an unjustified war of aggression by Russia, has killed thousands, displaced millions and destroyed cities, while the curtailment of food and energy exports has affected countries across the world.

Britain's Prime Minister Boris Johnson said Britain was willing to assist with demining operations off Ukraine's southern coast and was considering offering insurance to ships to move millions of tonnes of grain stuck in the country.