China on Friday said it would be "difficult" for it to take part in a planned conference on the war in Ukraine in June if Russia did not attend.

Ukraine is trying to whip up attendance at the peace conference in Switzerland, where it hopes to win broad international backing for its vision of the terms needed to end the war.

Ukrainian President Volodymyr Zelensky has urged China to take part but Beijing insisted any summit would need the participation of Russia, which Ukraine has rejected.

Beijing believes the conference "should have the recognition of Russia and Ukraine, equal participation of all parties and fair discussion of all peace plans", foreign ministry spokeswoman Mao Ning told a regular news conference.

"Otherwise, it is difficult for the conference to play a substantive role in restoring peace," she said. (AFP)