KIEV, Ukraine (AP) – Rescuers were searching Thursday for dozens of people trapped under the rubble of an apartment building destroyed in a nighttime explosion that killed at least 18 people in southern Ukraine, officials said.

The blast, apparently caused by leaky oxygen canisters, tore through the five-story building Wednesday night in the Crimea peninsula resort of Yevpatoriya, the Emergency Situations Ministry said.

“There was a loud bang, and we rushed out and saw half of the building was missing,” witness Sergei Yurchenko told Russian state television Channel One. “There were screams, and a child was crying.”

Up to 700 military rescuers were picking through mounds of concrete and glass to search for survivors in the wreckage, ministry spokesman Ihor Krol said. At least 21 people have been pulled out alive, with four of them hospitalized.

Once an hour the rescuers were standing silent to listen for possible cries for help.

At least 18 people were killed, including two children, said Volodymyr Ivanov, a spokesman for the Emergency Situations Ministry’s branch in the Crimea.

He said officials believe the explosion was likely caused by a leak from oxygen canisters in the building’s basement, but he did not elaborate on why the canisters were being stored there.

The blast decimated 35 apartments in the middle of the building, leaving apartments on either side exposed with walls missing, Krol said. It occurred around 9:45 p.m., when most of the destroyed section’s 62 registered inhabitants would have been home.

“This is a terrible, irretrievable loss for all the relatives and those close to those who perished,” President Viktor Yushchenko said in a statement. He announced a nationwide day of mourning on Friday.

Yushchenko and Prime Minister Yulia Tymoshenko both flew to Yevpatoriya to inspect salvage efforts and talk to survivors. Tymoshenko said in televised remarks that survivors would be given free housing before year’s end.

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