Staffan de Mistura says the cease-fire reached by the United States and Russia and set to begin at midnight Saturday in Syria gives the two world powers the task of making sure that everyone else abides by it, too.

The agreement does not apply to the Islamic State group or the al-Qaida-linked Nusra Front. De Mistura emphasized the U.N. considers them terrorist groups, and “no one else.”

Russia, a strong backer of Syrian President Bashar Assad, has faced claims that its airstrikes have hit opposition groups that Moscow considers terrorist groups, but the U.N. does not.