Ukrainian President Volodymyr Zelensky (right) speaks with a frontline soldier. File photo

The visit comes at a decisive moment in Russia's six-month invasion, with Ukraine routing Moscow's forces from swathes of the east and seriously undermining the Kremlin's ambitions to capture the entire Donbas region of Ukraine.

"Our blue-yellow flag is already flying in deoccupied Izyum. And it will be so in every Ukrainian city and village," Zelensky said in a statement on social media.

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"We are moving in only one direction — forward and towards victory."

Pictures distributed by his office showed the Ukrainian leader wearing dark-green and flanked by guards as he took selfies with soldiers and thanked troops at a flag-rising ceremony.

Ukraine has claimed sweeping successes in the northeastern Kharkiv region that borders Russia in recent days and also says it has clawed back territory along a southern front near the Kherson region on the Black Sea.

Zelensky said on Wednesday that Russia's occupation of Crimea — annexed by Russia in 2014 — was a "tragedy" and promised that his forces would eventually recapture the peninsula.

Kyiv says that since the beginning of September its forces have retaken hundreds of villages, towns and cities that were captured by Russian forces that invaded Ukraine on February 24.

Agence France-Presse