Ukraine is unlikely to launch a counter-offensive in its war with Russia before mid-June, when it hopes to have received more weapons from its allies, an adviser to President Volodymyr Zelenskyy said on Thursday.

Political adviser Oleksiy Arestovych also said he did not expect Russia's offensive in Ukraine to produce any "significant results" by May 9, when Russia celebrates victory over Nazi Germany in World War Two.

He said fighting continued at the Azovstal steel works where Ukrainian fighters and some civilians are holed up in the city of Mariupol but gave no details.