Ukraine's President Volodymyr Zelensky accused North Korea on Sunday of sending troops to Russia's army.

He was speaking days after visiting several European capitals to press his case for more military and financial help in the war against invading Russian forces.

"We see an increasing alliance between Russia and regimes like in North Korea," Zelensky said in his evening address.

"It is no longer just about transferring weapons. It is actually about the transfer of people from North Korea to the occupier's military forces.

"Obviously, in such circumstances, our relations with our partners need to be developed. The frontline needs more support," he said.

Zelensky completed a whirlwind two-day tour of European capitals last week including Berlin, London and Paris to ask for sustained military aid as Russia's invasion of Ukraine grinds on through its third year.

"When we talk about more long-range capability for Ukraine and more decisive supplies for our forces, it is not just a list of military equipment," Zelensky said.

"It is about increasing the pressure on the aggressor, which will be stronger for Russia than they can withstand. And it is about preventing a bigger war."