For the first time, the General Assembly will be required "to hold a debate on the situation" that sparks a veto in the Security Council within 10 working days.
A U.N. Security Council vote on a U.S. bid to extend an arms embargo against Iran failed last week. Now the U.S. secretary of state wants to bring back all sanctions lifted by the Iran nuclear deal.
Antonio Guterres laid out a sobering view of the current state of the world Tuesday, saying, "A wind of madness is sweeping the globe" as instability erupts into violence.
De Mistura told the U.N. Security Council he was leaving for "purely personal reasons." In his final month, he plans to make a major push to try to lay the groundwork for a new constitution in Syria.
"They do not want me or us to win because I am the first president to ever challenge China on trade, and we are winning on trade," the president said at a meeting of the U.N. Security Council.
Meanwhile, President Trump is weighing a possible military response. On Monday, he said "nothing is off the table" and has vowed to respond "forcefully."
Airstrikes, shells and barrels filled with TNT are being dropped on neighborhoods where civilians have no way to escape. More than 120 of those killed in the past week are children.
The Council's other 14 members approved the resolution, which didn't mention the U.S. by name. U.S. Delegate Nikki Haley called it "an insult" that "won't be forgotten."
"The states in the Security Council don't want justice," Carla Del Ponte told the Swiss publication Blick. "I can't any longer be part of this commission which simply doesn't do anything."