At least two people were killed and three injured in a powerful explosion in an apartment building in Ukraine Monday, the State Service for Emergencies said.The blast struck the nine-storey building at around 1.05 p.m., destroying three upper floors and sending debris crashing down over an area of about 30 meters in Nikolaev city, Xinhua reported.Ten people have been rescued and emergency crew were looking for more survivors.The cause of the blast remained unknown and authorities suspected a gas leak could have caused the explosion.