Regime says it will walk if "serious" discussions don't begin by Saturday.
U.N. mediator is to hold separate talks with rival delegations to assess their willingness to meet together.
Neither side appeared prepared to abandon their positions.
Damascus has denied claims that the regime has systematically tortured and executed about 11,000 detainees since the start of Syria's civil war.
U.N. chief's decision to invite Iran - only to withdraw the invitation a day later - proved a diplomatic fiasco.
Move prompts warnings from Syria's main opposition group and the United States.
Ceasefire plan for Aleppo offered as oppo weighs attending talks.
U.N. body says security concerns have slowed down the process.
Western intelligence agencies have visited Damascus for talks on combating radical Islamist groups, Syria's deputy foreign minister has told the BBC.
The Syrian government has retaken territory around the northern city of Aleppo, the military said on Tuesday, after two weeks of rebel infighting that has weakened the insurgency against President Bashar al-Assad.
