Nato has begun meeting in special session after Syria shot down a Turkish plane - an act condemned by Turkey as a "serious threat” to regional peace.In a letter to the UN Security Council, Turkey described the incident as a "hostile act by the Syrian authorities against Turkey’s national security”.Syria insists the F-4 Phantom jet was shot down inside Syrian airspace.Meanwhile, Syrian opposition activists have reported fighting near Republican Guard positions in suburbs of Damascus.There were clashes in Qadsaya and al-Hama, around 8km (5mi) from the centre of the city, the Syrian Observatory for Human Rights told AFP news agency. The British-based organisation also said that security forces had entered the Barzeh area of the city.Several casualties were reported in Qadsaya, according to the Local Co-ordination Committees.International media cannot report freely in Syria and it is impossible to verify the reports.Unknown intruder In its letter to the Security Council, Ankara said the shooting down of its F-4 reconnaissance plane was "a serious threat to peace and security in the region”.The BBC’s Barbara Plett at the UN says the letter does not ask the council to take any action.