Ukrainian President Volodymyr Zelensky told CNN's Wolf Blitzer on Thursday that Ukraine is unwilling to cede any of its land to Russia, standing firm that a concession of Ukrainian territory won't be part of any diplomatic negotiations to end the war.

"Ukrainians are not ready to give away their land, to accept that these territories belong to Russia. This is our land," Zelensky said in an exclusive interview aired Thursday on CNN's "The Situation Room."

"We always talk about that, and we are intending to prove it," he added.

Zelensky spoke to CNN at the same time as one of his top Western allies, British Prime Minister Boris Johnson, announced he would be resigning. Speaking in Ukrainian via a translator, Zelensky said he was confident that Britain's policy toward Ukraine "will not be changing" even if the country's leadership is in tumult.

"He resigned not because he was in Ukraine. I think on the contrary, what Johnson has been doing for Ukraine was helping us a great deal. I consider him a friend of Ukraine, but I think his society also supported Ukraine in Europe. That's why I think the UK, it's on the side of good, on the side of Ukraine," Zelensky said.

"And I'm sure the UK policy toward Ukraine is not ... changing because of Boris Johnson's resignation. Our relations obviously gained a lot from Boris Johnson's understanding of things. We went through a lot of dramatic moments quite quickly. The help we needed was delivered rather quickly ... if (his resignation) will affect this speed of help I don't know. I will pray to God it won't be affecting that help."

Russia's war with Ukraine has now lasted for more than four months, with no sign of either side backing down soon. Ukraine's early successes forced Russia to scale back its initial aims of toppling Kyiv, and Moscow's forces have now focused on taking territory in eastern Ukraine. Russian forces have now occupied most of the Luhansk region, outside of a few pockets of resistance, and are pressing toward cities in Donetsk.