Twenty-seven residents were pulled out alive from the building in the Black Sea resort of Yevpatoria in the Crimea peninsula.
The blast on Wednesday night, probably caused by canisters of oxygen stored in the basement, left rubble several metres (yards) high that was strewn with wires, smashed furniture, children's teddy bears and shoes.
"As I was walking by, I heard a bang and then I saw this building crumble," said one witness, who declined to be named.
Television footage showed rescue workers dragging out a man from underneath a heavy slab.
From time to time mobile phones were heard from under huge piles of debris.
President Viktor Yushchenko and Prime Minister Yulia Tymoshenko set aside long-running political feuding to arrive in the town together.
Yushchenko declared Friday a national day of mourning.
Tymoshenko promised families of the dead compensation and said they would be rehoused by the end of the year -- before Orthodox Christmas which Ukrainians celebrate on Jan. 7.
"This is a huge tragedy. The government and local authorities have joined forces to help the suffering families," she told a joint news conference with Yushchenko.
The two former allies patched up a governing coalition between their parties in parliament earlier this month.
Russian President Dmitry Medvedev offered condolences to relatives of the dead. Russians make up more than half the population of Crimea, an autonomous region which used to belong to Russia.
Medvedev said Russia's Black Sea Fleet, stationed in the Ukrainian port of Sevastopol and at the centre of a diplomatic spat this year between Moscow and Kiev, was ready to assist in rescue efforts.
Casualties caused by gas blasts in often crumbling apartment buildings are common occurrences in former Soviet republics, particularly in winter when residents use more heating.
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