Damascus government delegates to a Syrian Constitutional Committee left the second UN-sponsored round before it began on Monday, in what opposition members said was a stalling tactic, and it was unclear whether talks would take place on Tuesday.
But experts question whether President Bashar Assad will be willing to cede much in any negotiations after his Russian- and Iranian-back forces recaptured large areas of the country in offensives against rebels and militants since 2015.
Syrian television reported that the government delegation left the United Nations in Geneva on Monday because they did not get answers to their proposal to specify a work schedule.
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UN Special Envoy Geir Pedersen met the two co-chairs from the government and opposition sides and was continuing his consultations with a view to the panel's resumption, a UN spokeswoman said.
"The situation is clearly blocked," a Western diplomat said.
The opening round of the first Syrian peace talks in more than a year went "better than most people would have expected", Pedersen told reporters on November 8 after an opening 10-day round.
Forty-five delegates who form the committee' drafting group arrived at the UN European headquarters on Monday. The group is made up of 15 members each from the government, opposition and civil society but they did not meet together.
"The Constitutional Committee of today was not held. And the reason is there is no agreement on the programme or the schedule to be discussed for the meeting," Yahya al-Aridi, a Syrian opposition spokesman, told reporters upon leaving.
"The joint head of the Constitutional Committee from the regime side presented an item which is considered by him to be a schedule, namely it included fighting terrorism, it included lifting of sanctions and the condemnation of what he called the Turkish invasion," Aridi said, describing the government demands as "political".
Opposition delegate Bassma Kodmani said her side had proposed an agenda last week for a structured discussion but had had no reply.
"Now the government delegate comes with an agenda saying they want to discuss 'basic patriotic principles' as a set of pre-conditions to the constitutional discussion," Kodmani told Reuters.
The opposition was willing to discuss such principles as part of the constitutional work but not outside it, she said.
"The approach they (the government) suggest is designed to buy time," Kodmani said.