Indirect peace talks aimed at resolving Syria's five-year conflict have started but without the main opposition group.

The main opposition delegation has said it will not join the talks unless the government and Russia stop 'bombarding civilians' and sieges are lifted in rebel-held areas.

The meetings are part of a process outlined in a UN resolution last month that envisages an 18-month timetable for a political transition in Syria.

The first meeting is between the UN Special Envoy to Syria Staffan de Mistura and a government delegation headed by the country's ambassador to the UN, Bashar Ja'afari.

Syria's conflict has killed more than 250,000 people.