Russia is trying to create new "pseudo-republics" in Ukraine to break his country apart, President Volodymyr Zelenskyy said in his nightly address to the nation on Saturday.
Zelenskyy called on Ukraine's regions, including Kherson, which was captured by Russian forces, not to repeat the experience of Donetsk and Luhansk.
Pro-Russian separatists began fighting Ukrainian forces in those eastern regions in 2014.
"The occupiers on the territory of the Kherson region are trying to repeat the sad experience of the formation of pseudo-republics," Zelenskyy said.
"They are blackmailing local leaders, putting pressure on deputies, looking for someone to bribe." City council members in Kherson, a southern city of 290,000, on Saturday rejected plans for a new pseudo-republic, Zelenskyy said.
Russia recognised the Donetsk People's Republic and the Luhansk People's Republic before invading Ukraine in February.
Moscow said it had to protect the separatist regions, and is demanding that Ukraine recognize their independence too.
"Ukraine will stand this test. We need time and strength to break the war machine that has come to our land," Zelenskyy said.
Protection for the two eastern regions where pro-Russian separatists began fighting Ukrainian forces in 2014 was used as a pretext by Russia to start the invasion.
Kherson, a vital Black Sea port of 290,000 residents, was the first major city to fall earlier this month.
Russia bombarded cities across Ukraine on Saturday, pounding Mariupol in the south, shelling the outskirts of the capital, Kyiv, and thwarting the efforts of people trying to flee the violence.
In Mariupol, which has endured some of the worst punishment since Russia invaded, efforts to bring food, water and medicine into the port city of 430,000 and to evacuate civilians, were prevented by unceasing attacks.
More than 1,500 people have died in the city during the siege, according to the mayor’s office, and the shelling has even interrupted efforts to bury the dead in mass graves.
“They are bombing it (Mariupol) 24 hours a day, launching missiles. It is hatred. They kill children,” Mr Zelensky said during a video address.
