Amid a polio outbreak in war-torn Syria, the UN has launched the largest-ever vaccination campaign in the Middle East, aiming to immunise more than 20 million children in seven countries, officials said on Friday. "The polio outbreak in Syria is not just a tragedy for children; it is an urgent alarm -- and a crucial opportunity to reach all under-immunised children wherever they are," Peter Crowley, who heads the UN children's agency's polio division, said in a statement.