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. By Sunday, Ukraine’s figure had tripled to 3,000 sq km, before rising again to 6,000 sq km.
The precise scale of Ukraine’s gains has not been verified by the BBC.
Mr Zelensky vowed to take back all Ukrainian territory still occupied by Russian forces – he said he did not know when this would happen, but that “truth is on our side”.
Meanwhile, the Russian military is deploying so-called barrier troops in Ukraine to prevent its own units from fleeing, according to Ukrainian military intelligence.
Ukrainian defence officials said that according to intercepted conversations, “panic” and “refusals to fight” are setting in among Russian troops.
Elsewhere in Ukraine, officials accused Russian forces of targeting civilian infrastructure in the southern city of Kryvyi Rih on Wednesday.
Cruise missiles fired at the city’s “hydraulic structures” caused water levels in a nearby river to rise and threatened local drinking supplies, Ukrainian authorities said.
Responding to the attack, Mr Zelensky described Russia as a “terrorist state” that was trying to “flood Kryvyi Rih”.
“All the occupiers can do is to sow panic, create an emergency situation, try to leave people without light, heat, water and food. Can it break us? Not at all. Will they face a fair response and retribution? Definitely yes,” the Ukrainian president said.