President Vlodomyr Zelenskyy said Ukraine had recaptured roughly 6,000 square km of territory, double what officials had cited on Sunday. A sliver of Ukraine's land mass of around 600,000 square km, it is approximately equivalent to the combined area of the West Bank and Gaza.
In a video address, Ukraine's President Volodymyr Zelenskyy called on Ukraine's allies to "strengthen cooperation to defeat Russian terror."
Speaking in Balakliia, a crucial military supply hub taken by Ukrainian forces late last week, Ukraine's Deputy Defense Minister Hanna Malyar said 150,000 people had been liberated from Russian rule in the area. She spoke in the central square, where Ukrainian flags had been raised.
"The aim is to liberate the Kharkiv region and beyond - all the territories occupied by the Russian Federation," she said on the road to Balakliia, which lies 74 km (46 miles) southeast of Kharkiv, Ukraine's second largest city.
In a village northwest of Balakliia, resident Tetiana Sinovoz had tears in her eyes as she explained how the Ukrainian troops freed them from seven months of occupation.
"We thought there would be no village left, but we came out and the village was whole!" she said in front of what she said was the only building destroyed in what had sounded like a brutal battle, the school which the Russians had occupied.
Since Moscow abandoned its main bastion in northeastern Ukraine on Saturday, marking its worst defeat since the early days of the war, Ukrainian troops have recaptured dozens of towns in a stunning shift in battleground momentum.
Malyar said Ukrainian forces were consolidating their gains by checking for sabotage groups. The military said Russian forces were shelling parts of Kharkiv region retaken by Ukraine and attacking further south in Donetsk region, which Moscow is trying to seize for separatist proxies.
A senior U.S. military official said earlier that Russia had largely ceded territory near Kharkiv in the northeast and pulled many of its troops back over the border.
A video issued by Ukraine's border guards service showed what it said were Ukrainian troops liberating the town of Vovchansk near the country's border with Russia, burning down flags and tearing down a poster saying "We are one with Russia".
Fighting was still raging elsewhere in the northeastern Kharkiv region, she earlier told Reuters, saying Ukraine's forces were making good progress because they were highly motivated and their operation well planned.
Ukraine had repelled the Donetsk region attacks, its general staff report said, while Denis Pushilin, head of the separatist Donetsk People's Republic, said its forces were repelling Ukrainian attacks and he believed the situation would improve.
Serhiy Gaidai, Ukrainian governor of the Luhansk region, which Moscow has seized, said he expected a major Ukrainian offensive there.
Reuters could not immediately verify the battlefield reports.
U.S. Secretary of State Antony Blinken said on Monday Ukrainian forces made "significant progress" with Western support to ensure it has the equipment it needs.
Since Russia's Feb. 24 invasion, Washington and its allies have provided Ukraine with billions of dollars in weapons that Kyiv says have helped limit Moscow's gains. Russian forces control around a fifth of the country in the south and east. Ukraine is now on the offensive in both areas.
Ukraine's foreign ministry singled out Germany, saying in unusually blunt language that it was disappointing that Berlin had not provided Leopard tanks and Marder infantry fighting vehicles.
On Monday, German Defense Minister Christine Lambrecht rejected sending tanks "unilaterally". Some saw the remarks as leaving open the possibility that Berlin could do so as part of a pan-European consortium.
The German foreign ministry did not immediately respond to a request for comment.
Criticism of Russia's leadership from online nationalist commentators demanding mobilization was an example of "pluralism", Peskov told reporters, adding that Russians as a whole continued to support Putin.
Kremlin spokesman Dmitry Peskov said there was no discussion of a nationwide mobilization within Russia to bolster the operation in Ukraine, which he has said will continue until it achieves its goals.