Pro-Russian authorities in the Ukrainian region of Kherson, which Russia claims to have annexed, urged those remaining in the north of the province to leave "immediately" in the face of Kyiv's advancing counteroffensive on Saturday.
The recommendation to evacuate came as Ukrainian President Volodymyr Zelensky said Russia launched 36 rockets overnight in a "massive attack" on Ukraine, following reported strikes on energy infrastructure that resulted in over 1 million people being left without power.
Kyiv's forces have been advancing along the west bank of the Dnipro River and are now believed to be closing in on the Kherson region's main city, which is also called Kherson.
The first major Ukrainian city to fall to Russian troops in February, Kherson would be a major prize in Ukraine's counteroffensive should its forces succeed in recapturing it.
In recent days, Russia has been moving Kherson residents to the other side of the Dnipro River, in what it terms a civilian evacuation, but Kiev has described as "deportations."
"Due to the tense situation on the front, the increased danger of mass shelling of the city and the threat of terrorist attacks, all civilians must immediately leave the city and cross to the left bank of the [Dnipro] river," the region's pro-Russian authorities said on social media.
A Moscow-installed official in Kherson, Kirill Stremousov, told Russian news agency Interfax on Saturday that around 25,000 people had already made the crossing.