The death toll from an explosion that leveled an apartment building in southern Ukraine reached 26 on Friday as the country observed a day of national mourning, officials said, according to AP.

The search for survivors continued after rescuers pulled 21 people from under the mounds of concrete and glass following Wednesday`s huge blast at the five-story building in the Crimean peninsula resort of Yevpatoriya.

But officials do not know how many people could remain beneath the rubble and hold out little hope of finding anyone else alive with temperatures over the two nights since the blast reaching minus 6 degrees Celsius (21 degrees Fahrenheit).

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The entire central section of the building came down, exposing apartments on either side. Sixty-two people were registered inhabitants of the destroyed section of the apartment and rescuers paused their work every hour to listen for cries for help. Two children were among those killed, officials said.

Friday was designated a day of national mourning in Ukraine, with entertainment events canceled and flags lowered to half-mast.

Officials believe oxygen canisters in the building`s basement exploded, said Volodymyr Ivanov, a spokesman for Crimea`s branch of the Emergency Situations ministry.

Ivanov said one canister was found intact and sent to forensic investigators. Ivanov said that other possible causes had not been ruled out, without elaborating.

Ukrainian President Viktor Yushchenko and Prime Minister Yulia Tymoshenko both flew to Yevpatoriya to inspect rescue efforts and talk to survivors on Thursday. Tymoshenko said in televised remarks that survivors would be given free housing before year`s end.

Yushchenko thanked Russia for offering to send naval personnel from Russia`s Black Sea fleet, stationed at the Ukrainian port city of Sevastopol, to help with the rescue, but said they were not needed.

Neglect of safety precautions has led to frequent explosions in apartment buildings and public facilities in the ex-Soviet nations.