A cease-fire in Syria negotiated by Russia and the U.S. is set to take effect on Monday at sundown, and many of the main warring factions — the government of President Bashar al-Assad, its allies Iran and Hezbollah, and the opposition Free Syrian Army — said they will abide by the truce. The Islamist rebel group Ahar al-Sham said it will not participate, calling the agreement too favorable to Assad’s government. Underscoring doubts about the cease-fire, dozens of people were killed in government bombing raids over the weekend — Al Jazeera puts the number at more than 100, including 55 civilians at a market in Idlib. Rebel groups attacked, too. Analysts say such jockeying for position is normal before cease-fires, but previous truces have collapsed within weeks.