The meeting follows weeks of difficult negotiations in preparation.
Staffan de Mistura, the UN's special Syria envoy, said on Wednesday he was "not expecting a breakthrough".
The opposition is insisting that the fate of President Bashar al-Assad is on the agenda - something the government has refused to discuss.
At least 300,000 people have been killed since the war began in 2011. More than 4.8m have fled the country and a further 6.3m have been displaced inside Syria itself.
