President Volodymyr Zelensky has been to the recaptured city of Izyum, a key logistics hub in north-eastern Ukraine.
During his visit, Mr Zelensky thanked troops who took part in the counter-attack against Russian occupiers.
He oversaw a flag-raising ceremony and said the Ukrainian flag would return to every city and village in the country.
Ukrainian officials say they are targeting towns in the eastern Donbas region after making a series of gains in a rapid counter-offensive.
In recent days, Ukraine’s army has reclaimed swathes of occupied territory, forcing Russian troops to retreat.
In a late evening address on Tuesday, Mr Zelensky said his forces were fortifying their hold over 8,000 sq. km (3,088 sq. miles) of retaken territory in the Kharkiv region.
Last Thursday, President Zelensky said Ukrainian forces had retaken 1,000 sq. km, but by Sunday, that stated figure had tripled to 3,000 sq. km.
The precise scale of Ukraine’s gains has not been verified by the BBC.
He vowed to take back all Ukrainian territory still occupied by Russian forces – he said he did not know when this would happen, but that the “truth is on our side”.
US President Joe Biden said Ukraine had made “significant” gains.
Mr Biden said it was “clear” that the advance had enjoyed success, but cautioned that the offensive “could be a long haul”.
While Russia still controls around a fifth of Ukraine’s territory, towns in the Donbas that fell early in the war are now the focus of Kyiv’s advancing forces.
After failing to capture cities across the country, including the capital, Kyiv, Russia has been focusing on the Donbas, parts of which were already under the control of Russian-backed rebels before Russia launched its invasion this year.
Andrey Marochko, military commander of the self-declared Luhansk People’s Republic – one of two regions that make up the Donbas – told Russian state media that fighting had reached the borders of the territory.
And Serhiy Hayday, the exiled Ukrainian head of the Luhansk region, said Ukrainian forces were engaging Russian troops on the outskirts of Lyman. -BBC