The UN mediator at Syria peace talks in Geneva has "undermined" his position by appealing to Moscow to convince Damascus to hold new elections, the Syrian government's top negotiator said Thursday.

Bashar al-Jaafari harshly criticised UN envoy Staffan de Mistura for comments made to Swiss television Wednesday calling on Russian President Vladimir Putin to "have the courage" to convince his Syrian counterpart to allow new polls.

"His statement undermined his mandate as a facilitator of the talks, which will affect the entire Geneva process," he told reporters as the eighth round of negotiations concluded with no sign of progress.