National Security and Defence Council chief Andriy Parubiy said the armed forces would expand the "active stage of the operation in other towns where extremists and terrorists are carrying out illegal activities".
AFP reporters near the eastern town of Kostyantynivka, where rebels seized the town hall on April 28, saw a pro-Russian checkpoint abandoned and smouldering and barricades being hastily erected in the centre.
Rebels defending the town hall behind makeshift barriers told AFP there had been fighting overnight near the town's television tower.
Central Slayvansk appeared relatively calm this morning but citizens reported increasing difficulty obtaining basic foodstuffs in the besieged town of 160,000 people.
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Meanwhile, seven European inspectors from the Organisation for Security and Cooperation in Europe arrived home last night after an eight-day ordeal in captivity, a small chink of light in the worst East-West crisis since the Cold War.
Interim president Oleksandr Turchynov declared two days of mourning yesterday after brutal violence in Odessa claimed 42 lives and at least 10 died in military operations around Slavyansk.
The city was still reeling from horrific violence on Friday when deadly clashes between the two sides culminated in a building fire that left 38 dead, most overcome by fumes, others from jumping from windows in a desperate bid for survival.