Add a potential polio epidemic to the threats that innocent civilians now face because of Syria’s civil war. It is part of what American officials say may be the worst humanitarian disaster since the 1994 Rwandan genocide that killed an estimated 800,000 people. But while the tragedy is unfolding in full view, many countries, including Russia and China, have given hardly anything to the United Nations campaign to meet the Syrians’ basic needs.

Civilians have paid a terrible price ever since President Bashar al-Assad of Syria used force to crush peaceful protests that began in 2011, touching off a full-scale civil war. Officials now put the death toll, including combatants, at 115,000.

Of the Syrians who have survived the war so far, some five million are virtual refugees in their own country — trapped in neighborhoods isolated by military blockades, or uprooted from their homes and living in vacant buildings, schools, mosques, parks and crowded homes of relatives. Most are desperately short of food and medicine, a deprivation likely to worsen as winter sets in.

Meanwhile, another two million Syrians have fled to Turkey, Jordan, Lebanon, Iraq and Egypt, meaning that seven million people, or about one-third of Syria’s population, have seen their lives upended by the war.