It's the first claim of progress since Ukraine began its counteroffensive two weeks ago in the south of the country.
Ukraine claims it has recaptured around 500 square kilometres of territory from Russian forces during a two-week counteroffensive.
It is the first quantified estimate of Kyiv's progress in the south of the country.
"Our successes in the last two weeks are quite convincing," Natalia Gumenyuk, military spokeswoman for the South, told reporters on Monday.
She said Ukraine had "liberated about 500 km2" including the localities of Vysokopillia, Bilogrka, Sukhy Stavok and Myroliubivka.
Euronews is unable to independently verify these claims.
On Monday, Russia said it would continue what it insists on labelling as its "military intervention" in Ukraine "until the objectives are achieved", said Kremlin spokesman Dmitry Peskov.
It comes after major gains for Ukraine in the northeast of the country, where, amid reports that Russian soldiers were surrendering en masse, a spokesman for the office of the Ukrainian president said on Monday that there were so many prisoners of war that Ukraine was running out of space to accommodate them.
The British Ministry of Defence (MoD) said Ukraine had retaken territory double the size of Greater London since last Wednesday.
"The rapid Ukrainian successes have significant implications for Russia’s overall operational design," the ministry wrote. "The majority of the force in Ukraine is highly likely being forced to prioritise emergency defensive actions."
"Ukrainian forces have penetrated Russian lines to a depth of up to 70 kilometres in some places and captured over 3,000 square kilometres of territory in the past five days since September 6 - more territory than Russian forces have captured in all their operations since April," said the Institute for the Study of War in its latest assessment.
Russian forces, the Institute said, "are not conducting a controlled withdrawal and are hurriedly fleeing southeastern Kharkiv Oblast to escape encirclement around Izyum".
Video clips on social media show scores of Russian military vehicles, including tanks and armoured personnel carriers, abandoned by the side of the road as their crews seemingly fled ahead of the Ukrainian surge.
Other videos show grateful civilian residents, liberated after six months under Russian occupation, crying and hugging Ukrainian soldiers who advanced so far, so fast, they were able to reach the town of Hoptivka, just 2km from the Russian border.
So what happens next? The Institute for the Study of War predicts that Ukrainian forces "will likely capture" Izyum within the next 48 hours, which they say would mark the most significant Ukrainian victory since winning the Battle of Kyiv in March.
A Russian announcement about troops "regrouping" as part of a "controlled withdrawal" has been dismissed as a false reframing of the situation.
Britain's Ministry of Defence says that the situation in Donbas is likely to erode the limited trust that Russian soldiers have in their senior military leadership.
The Institute for the Study of War says bluntly that the Ukrainian counteroffensive "is effectively paralysing the Russian occupation leadership that is likely afraid for its fate".