Formation operations are standard pre-offensive military practice and include strike weapon systems, command and control, ammunition depots, and other targets to prepare the battlefield for a planned offensive.
The US believes that the Ukrainian counter-offensive, which has long been expected, will include a combination of air and ground operations.
Ukraine said on Monday morning that action was ongoing.
“Yes, (Ukrainian forces) have launched offensive operations in several directions on the Southern Front to liberate the occupied territories,” Natalia Gumeniuk, a spokeswoman for Ukraine’s Operational Command South, told CNN, adding: “All details will be available after the operation is completed.”
The plans come after Russia’s war in Ukraine passed the six-month mark, a senior US official said, and US estimates indicate Russia was able to deploy fewer units to the front lines than originally thought.
The official said many of the existing units that Russia is organizing into combat tactical groups or BTGs, including infantry, tanks, artillery and air defenses, are underdeployed and some are even half the size of their normal strength. The US is also watching Ukrainian forces capitalize on US- and NATO-supplied HIMARS mobile missile launchers, which have enabled Ukraine to strike and destroy targets in Russian-held territory.
Neither Ukrainian nor US officials have disclosed the main targets of the expected counteroffensive. In recent weeks, Ukrainian forces have been advancing around the key southern city of Kherson, which is currently occupied by Russian forces. Kherson is considered crucial for control of the southern coast of Ukraine and access to the Black Sea.
Ukrainian nuclear operator Energoatom said on Friday that the station was reconnected to the Ukrainian power grid a day after the station went offline for the first time in its history.