Air and missile strikes on a school district and other areas in insurgent - held Damascus suburbs have killed at least 28 people, including two children and a school principal, a group monitoring the Syrian war says. The Syrian Observatory for Human Rights said unidentified war planes hit the town of Douma, 15 kilometres north-east of the capital, and surface-to-surface missiles fired by the Syrian army also hit the surrounding areas, including Eastern Ghouta. The areas have been under heavy bombardment in recent months. The Syrian army has said its strikes target insurgents that have launched attacks on government-held areas. The Observatory, which gathers information from a network of contacts on the ground, said at least 40 mortar bombs fired by insurgents in the eastern suburbs hit Damascus on Sunday, killing at least one child and wounding several. State news agency SANA said three civilians had been killed and at least 30 wounded in mortar attacks targeting residential neighborhoods of Damascus. Most of the victims were school students, it said, citing its reporter in the area. Syrian president Basher al-Assad said he would not negotiate with armed groups, appearing to scupper peace talks that Russia and the United States hope to bring about next month. Washington helped broker an agreement reached on Thursday by more than 100 members of Syria's opposition parties and more than a dozen rebel fighting groups to send a joint team to meet the government under UN auspices in January. Many of Douma's residents have fled the nearly five-year conflict, moving to nearby rural areas. Syria's civil war has killed more than 250,000 people and driven more than 10 million from their homes.