The Syrian regime on Thursday accused the main opposition delegation of taking the Syria peace talks "hostage" over its refusal to include terrorism on the agenda.
The peace talks in Geneva are the first under UN auspices in 10 months. They convened after Turkey, Russia and Iran brokered a shaky ceasefire in December between the warring parties in Syria.
"Progress in the Geneva round must not be held hostage by the Riyadh platform," lead Syrian negotiator Bashar al-Ja'afari told reporters, referring to the Saudi-backed Syrian opposition's High Negotiations Committee (HNC).
"The Riyadh opposition will be held responsible for any failure of the Geneva talks," Ja'afari said.
The two sides have brought different priorities to the table. The regime wants terrorism on the agenda, and the opposition wants to talk about a roadmap for removing Bashar al Assad from power.
Thursday's escalation followed a day in which the talks appeared to gain momentum, when UN special envoy Staffan de Mistura reportedly told the opposition delegation that the regime was ready to discuss political transition.
Russian rhetoric dampens mood
However, Moscow's intervention dampened the mood in Geneva.
"The talks are once again raising questions about the ability of representatives of the Syrian opposition to do a deal," Russian Foreign Ministry spokeswoman Maria Zakharova told a news briefing in Moscow.
"The so-called High Negotiations Committee is refusing to cooperate equally with the Moscow and Cairo platforms and is in fact sabotaging fully fledged dialogue," she said, referring to two smaller opposition groups looking to Moscow for support.
Russia has sought to revive diplomacy since its air force helped the Syrian regime and allied militias defeat rebels in Aleppo in December, Assad's biggest victory in six years of war that has killed hundreds of thousands and displaced millions.
It has held parallel negotiations in coordination with Turkey and Iran in the Kazakh capital Astana to reinforce the December ceasefire. But violence has escalated since the Geneva talks started last week with representatives of the warring sides appearing no closer to substantive negotiations.
Discussions will continue on Friday with a focus on the issue of political transition in Syria, opposition delegation leader Nasr al-Hariri said late Thursday.
Al-Hariri said that the "political process is not an easy process, it cannot be concluded in one or two weeks, more effort is needed, more time is needed." He stressed that his delegation was willing to keep negotiating.