President Petro Poroshenko has vowed that militants-controlled lands in eastern Ukraine will return under Kyiv's control and ruled out the possibility of the east of the country becoming the site of a "frozen conflict."

"I'm sure that Ukraine will be sovereign and independent and nothing will threaten its territorial integrity. It won't be a frozen conflict because Donbas won't survive without Ukraine," Poroshenko told a briefing in Odesa.

He added that Ukraine's position in the conflict enjoys global support.

He expressed confidence that the armed forces and ordinary people would prevent adversaries of the Kyiv government from seizing Mariupol or Odesa.

"Today Odesa has become a pro-Ukrainian city… Neither Odesa nor Mariupol can be captured," he said.

Earlier on Thursday, the prime minister of the self-declared Donetsk People's Republic, Alexander Zakharchenko, promised that the republic would regain control of towns it had surrendered to the Ukrainian army. "Kramatorsk, Mariupol, Sloviansk - they will be ours. We are determined to take them back," Zakharchenko told reporters.

The chief spokesman for Ukraine's top security body ridiculed Zakharchenko's words.

"We have a good phrase in Ukraine: God grant that our calf can eat a wolf. Let them try," Andriy Lysenko, spokesman for the National Security and Defense Council, told a briefing in Kyiv.