Ukrainian President Volodymyr Zelensky declared that military forces have recaptured over 2,000 square kilometers of territory after a counteroffensive against Russian forces in the east of the country.
If Zelenskiy's statement is confirmed, it means that Ukrainian forces have more than doubled the recapture of territory in just over 24 hours, the BBC writes.
Ukraine's rapid advance saw troops enter the main towns of Izyum and Kupiansk on Saturday.
Officials in Kiev said Ukrainian forces were still fighting to gain control of a number of settlements around Izyum, adding that more than 30 towns and villages had been retaken in the Kharkiv region.
The Russian defense ministry later said its troops had been withdrawn from the city of Izyum to allow them to "regroup".
Also, the ministry confirmed the withdrawal of troops from another key town, Balaklyia, in order to "strengthen efforts" on the Donetsk front.