Russia declared war on Ukraine on Thursday following days of escalating tension between the two nations. As big explosions were heard in eastern Ukraine -- Kyiv, Kharkiv and Odessa -- Russian President Vladimir Putin vowed retaliation against any world leaders that tried to interfere in the matter between the two countries. Air radio sirens were heard in the morning at 7am in Kyiv. Ukrainian president Volodymyr Zelenskyy declared martial law, saying Russia has targeted Ukraine’s military infrastructure. He urged Ukrainians to stay home and not to panic.