There were about 35 flats in the building
Ukraine is mourning the victims of Wednesday's blast at a block of flats in the Black Sea resort of Yevpatoria that left at least 27 people dead.
Officials said 21 people had been rescued but a number of tenants remained missing.
Two entrances to the five-storey building were blocked by falling concrete following the blast.
The cause is unknown, although officials believe that gas cylinders may be to blame.
Day of mourning
Rescuers have been scrabbling through the rubble, calling for quiet to hear cries for help.
One resident, Lidia Kovalenko, told the AFP news agency: "I saw a blue flame and smoke and... a horrible explosion."
There were about 35 flats in the building, which was built in the 1960s.
On Thursday, Ukrainian President Viktor Yushchenko and his political rival, Prime Minister Yulia Tymoshenko, travelled in the same car to the site of the blast.
Mr Yushchenko later declared 26 December a national day of mourning.
Ms Tymoshenko said initial reports suggested dangerous materials were being stored without a proper permit in the basement.
Russian President Dmitry Medvedev expressed his condolences and said his Black Sea fleet was ready to help.
Yevpatoria is a popular summertime resort in Ukraine's Crimea peninsular.
Correspondents say casualties caused by gas explosions in often crumbling apartment buildings are common occurrences in former Soviet states, particularly in the winter, when residents use more heating.
One such blast, in October 2007, killed 23 residents in the central Ukrainian city of Dnipropetrovsk.