WASHINGTON - An influx of almost 1 million refugees from Syria into neighboring Lebanon poses a serious threat to the already fragile country, but donor nations may not grasp the potential impact of further destabilization, a UN official said on Thursday."There is not a single country in the world today that is shouldering as much in proportion to its size as Lebanon," said Ninette Kelley, regional representative for Lebanon for the Office of the United Nations High Commissioner for Refugees."If this country is not bolstered, then the very real prospect of it collapsing and the conflict of Syria spreading full force to Lebanon becomes much more likely," she said during a visit to Washington. Last month, top UN officials said that as Syria's grinding conflict enters its fourth bloody year, Syrians are set to replace Afghans as the world's largest refugee population.While hundreds of thousands of Syrians have also sought refuge in Jordan, Turkey and elsewhere, the largest concentration of Syrian refugees, close to 1 million people, can now be found in Lebanon, increasing the population of the tiny country by about a quarter, the United Nations said.The massive influx threatens to upset Lebanon's fragile demographic balance between Shi'tes, Sunnis, Druze and Christians, and comes as the country, which fought its only bloody civil war from 1975-1990, struggles to contain mounting violence seen as linked to the conflict next door.Earlier this week, Lebanon's foreign minister said that the crisis was "threatening the existence of Lebanon." This month, the Lebanese parliament gave a newly formed cabinet a vote of confidence, ending almost a year of political deadlock.A major challenge for the new government will be the mounting cost of the refugee crisis, which has strained public infrastructure as people fleeing violence in Syria seek housing, food, and healthcare at a time of economic slowdown in Lebanon. The challenge of educating refugee children provides one stark example. Kelley said that 400,000 Syrian refugee children in Lebanon require schooling, now outnumbering Lebanon's own 300,000 children in public schools.To help the country cope, Kelley said the United Nations is seeking to support informal education for refugee children who cannot attend overwhelmed Lebanese schools.But such activities will require additional, sustained funding from donors who are facing their own fiscal pressure and competing needs from elsewhere. Kelley said the United Nations and other aid providers were struggling to secure such funding.The United nations has estimated that $1.7 billion is needed for this year to help the United Nations, aid organizations, the Lebanese government and others to support refugees from Syria in Lebanon, and to mitigate the impact of the refugee crisis there.So far, pledges have been made for 14 percent of that amount, the UN said."Not everyone appreciates the small size of Lebanon and that 25 percent of its population are now refugees, most of whom came in a single year," she said.Lebanon is smaller than the US state of Connecticut. "I also don't think it's widely appreciated that, should Lebanon become destabilized, what that would mean in terms of how much more difficult that would be to form a solution inside Syria, the risk that could have to Israel's stability, the kind of ground that would provide to more militant actors," she said.The United States has been the largest single donor of assistance related to the conflict in Syria. The US State Department says that US humanitarian assistance across the region related to the conflict amounts to $1.7 billion to date.