British Prime Minister Boris Johnson was in Kyiv on Wednesday, hailing the "strong will of Ukrainians to resist" Russia's invasion, as the nation celebrates its Independence Day and marks the milestone of six months of war.

"There's a strong will of Ukrainians to resist. And that is what (Russian President Vladimir) Putin failed to understand," Johnson said in front of reporters during a surprise visit to the capital.

"You defend your right to live in peace, in freedom, and that's why Ukraine will win."

Johnson met with President Volodymyr Zelensky who awarded him the "Order of Liberty" -- a Ukrainian honour given to those who buttress the country's sovereignty.

"Not every country is lucky to have such a friend," said Zelensky.

"The UK helps to bring our victory closer, and we're sure of this victory," he added.

Britain has been a stalwart ally of Ukraine since Russia launched its large-scale invasion on February 24.

London has funnelled military hardware, funding and training resources to Kyiv's embattled forces, now waging battles in the nation's south and east.

"We in the UK, not for a second will we give up to Putin's blackmail," Johnson said.

Wednesday also marks the day Ukrainians annually celebrate their independence from the Soviet Union in 1991.