Members of pro-Russian self-defence units take cover behind cars outside a military base in the Crimean town of Belbek near Sevastopol March 22, 2014. Russian troops have surrounded a Ukrainian airbase in Crimea and issued an ultimatum to forces inside to surrender, the deputy commander of the base in Belbek, near Sevastopol, said on Saturday. REUTERS/Vasily Fedosenko (UKRAINE – Tags: POLITICS MILITARY) – RTR3I5D1