YEVPATORIA, Ukraine (AFP) - Nineteen people were killed and at least ten more were missing after an explosion ripped through an apartment block in a Ukrainian Black Sea resort town, local officials said Thursday. Rescuers and residents were desperately searching the wreckage for survivors after the explosion struck a five-story residential building in the Crimean Black Sea town of Yevpatoria late Wednesday night. "This is a horrible, irreparable loss for all the relatives and loved ones of those who died," said Ukrainian President Viktor Yushchenko, who declared December 26 a day of national mourning. Yushchenko and Prime Minister Yulia Tymoshenko, who have been engaged in a ferocious political row over the last months, arrived in Crimea and even headed to the scene in the same car, in a rare show of unity. Russian President Dmitry Medvedev expressed condolences over the blast, saying the country's Black Sea fleet was ready to assist in a rescue operation. Rescuers on the scene said Thursday afternoon 19 people were killed in the blast. A spokesman for the regional emergency situations ministry earlier put the casualty toll at 18 dead, including two children, and up to 15 missing. The spokesman said that 21 people had been rescued from the rubble so far. The search party were regularly maintaining periods of complete silence so they could hear the voices of those still trapped under the rubble, an AFP correspondent said. Two sections of the apartment building, constructed in 1965 and containing around 35 flats, were destroyed in the explosion. Tymoshenko, citing preliminary information, told reporters in the regional capital Simferopol that the accident was caused by an explosion of oxygen or acetylene cylinders in the building's basement. "According to preliminary expert conclusions, there was a workshop in the building's basement where explosive materials have been used without any kind of permission," she said. Earlier, authorities refused to confirm that it was a gas explosion, the usual explanation for such incidents in apartment buildings in the former Soviet Union. Ukraine's Emergency Situations Minister Volodymyr Shandra, reported earlier that the "catastrophe had happened because of an oxygen or carbide explosion." "All the repair and maintenance crews are storing their oxygen in basements. This is the practice all over Yevpatoria," a witness said in comments broadcast by Russian television. Yevpatoria residents said they had not smelled gas but heard a explosion. "There was no smell of gas," said Yevpatoria resident Svetlana Sergiichuk in comments broadcast on Russia's Channel 1. Another resident, Lidia Kovalenko, said: "I saw a blue flame and smoke and... a horrible explosion." "It felt like an earthquake but it must have been something else," said another resident. Witnesses said that the cries of those trapped under the rubble could still be heard overnight, giving rise to hopes that more survivors could be found. "We clearly heard a girl crying," one witness told AFP. The Ukraine emergencies ministry said it had some 300 rescuers already on the scene. Even mobile phones were being used in a bid to locate the survivors. Television pictures showed rescuers seeking to uncover survivors by pulling away the huge concrete blocks, which were all that remained of the section of the building that exploded. Yevpatoria is a popular summer resort town in Ukraine's Crimea peninsula, lying some 60 kilometres (40 miles) west of the regional capital Simferopol. Crimea has a Russian-speaking majority population as well as substantial Ukrainian and Tartar minorities. Explosions in ageing and poorly maintained Soviet-era apartment buildings, usually caused by gas, are not unusual in Ukraine and the former Soviet Union. In October 2007, 15 people were killed in a gas explosion that partially destroyed a nine-storey building in Ukraine's eastern city of Dnipropetrovsk.