Ukrainian presidential advisor Oleksiy Arestovych on Monday described as "successful" Ukraine's military operation in the key cities in the eastern Lugansk region.

"The defense of the Lysychansk-Severodonetsk agglomeration is a successful military operation," Arestovych wrote on Facebook.

He added that Ukraine has performed all four key tasks of the defensive operation, including interdicting key enemy forces, inflicting losses on the enemy forces, buying time for receiving weapons and creating conditions for offensive operations in other sectors of the frontline.

Currently, Ukraine is carrying out counterattacking actions in the eastern Donetsk region and in the southern Kherson and Zaporizhzhia regions, Arestovych said.

On Sunday, Russian Defense Minister Sergei Shoigu informed President Vladimir Putin of the control of the Lugansk region, according to Russia's Defense Ministry.

Late Sunday, Ukrainian President Volodymyr Zelensky confirmed that Ukrainian forces withdrew from Lysychanska, a key city of Lugansk, but vowed that Ukrainian forces "will return."