The revolt is really a civil war in which forces dominated by Islamist extremists such as Al Nusra (= Al Qaeda) tried to overthrow the legitimate secular government, a government that includes Sunni, Shia, Alawite, Christian, and others. There is no UN resolution or basis in law or morality for the US or other outside forces to intervene unless invited in by the government.
This is is not a clear-cut situation. Many Sunnis have fled East Aleppo and the Rebels, while not as vicious as Daesh, have committed their share of atrocities. There was a cease fire a few weeks ago and the US broke it by bombing a Syrian airbase and killing over a hundred. The Syrians and Russians have set aside times and opened humanitarian corridors so civilians can leave the city, but there are reports some rebels make it difficult to escape. We should also realize the Rebels are choosing to fight in an urban environment and that such warfare often leads to high numbers of civilian casualties. Some would characterize what they are doing as using civilians as human shields. Perhaps we could save more people by negotiating with the Syrians and Russians to set up demilitarized safe zones in return for stopping the spigot of US supplied arms that have kept this war going for 5 years.
Despite all the propaganda, the people of the US strongly oppose any deepening involvement or military intervention against the Syrian government. They do not want to get into a shooting match with the Russians and risk starting WWIII over Syria.