Syrian government forces and rebel forces traded salvos of rockets and mortar shells Thursday around Damascus, killing at least 21 people in tit-for-tat violence.
Outside observers said anti-government forces struck first, after a rebel commander vowed to hit government-held areas of the Syrian capital to avenge the past week's strikes by government warplanes on opposition-held suburbs. That commander, Zahran Alloush of the Army of Islam rebel group, said in a tweet that his forces would keep firing mortars and rockets "until the capital is cleansed."