( CNN ) -- Of all the terrible conflicts facing the world in 2013 , Syria is undoubtedly the most complex and dangerous . Violence has left four million people inside Syria in desperate need of help -- shelter , food , education , clean water , health care and protection -- and has uprooted two million inside the country and sent 600,000 fleeing the horrors of war into neighboring countries . Now a bitter winter is the new enemy . Syria 's children suffer the most . At least half of all those affected by the conflict are children . Too many have been injured or killed ; too many have seen family and friends die , their homes and schools reduced to rubble . The good news is our aid is reaching approximately 1.5 million Syrians , even in areas of fighting -- children are being vaccinated , and temporary schools are being set up , families are being fed and sheltered -- thanks to our work and to the valiant , efforts of many partners like the Syrian Arab Red Crescent . But we could do so much more . There are areas inside Syria where our ability to deliver is intermittent at best ; where we can not reach those in need of our help . We appeal to all the parties involved in the conflict to grant unrestricted humanitarian access inside Syria . Sadly , if this appeal continues to go unheeded , we fear the already horrific level of suffering will become even worse . Crisis in Syria : The Refugees . With each passing day , and each passing week it becomes harder for Syrians to endure . Most can not flee and find safe havens in neighboring countries . Some find precarious refuge with friends or the kindness of strangers in another town . Others shelter in abandoned , unheated buildings or makeshift camps . Many find themselves moving from one place to another , again and again as the conflict spreads . Living conditions in all areas of the country are deteriorating rapidly . It is not only the violence that people fear , but the combined threat of hunger , cold and illness . Neighboring countries have opened their borders to 600,000 Syrian refugees and , with the help of humanitarian organizations like ours , offer basic support for their survival . But even they face difficult challenges . Most refugees are children who have escaped with mothers and grandmothers . Now , many have been refugees for 21 months . It has been UNHCR 's job to register them and provide them with shelter and basic relief items like mattresses , blankets and kitchen sets . What 's next for Syria in 2013 ? In most places , WFP vouchers allow them to buy fresh food from the market . UNICEF helps children overcome their trauma , gets them into schools , and gives them with books and supplies and access to better health . Host communities open their homes and their hearts . Host governments provide medical and other community services . As the numbers of refugees grow , so does the strain on these host governments . The resources provided by Lebanon , Jordan , Turkey and Iraq are dangerously stretched . No one can predict how long this will last . What is needed now is support from the entire international community to asylum countries and organizations like ours to help us do more . In December , the U.N. appealed for $ 1.5 billion for the humanitarian response both inside and outside Syria and we are urging donors to contribute more . If the conflict ca n't be stopped now , the least we can do is ease the suffering . The opinions expressed in this commentary are solely those of António Guterres , Ertharin Cousin and Anthony Lake .
Four million people inside Syria are in desperate need of help . More than half a million others have fled to neighboring countries . U.N. agencies say they struggle to get aid to some areas inside Syria . U.N. : `` Living conditions in all areas of the country are deteriorating rapidly ''