The UN's Syria envoy on Monday put more pressure on the regime to make concrete proposals on political transition, as Damascus again ruled out any negotiations about the fate of President Bashar al-Assad.
The apparent impasse on the crucial issue of forming a transitional Syrian government came as peace talks in Geneva entered a second week, with United Nations mediator Staffan de Mistura underscoring the need to make gains before negotiations pause on Thursday.
Speaking to reporters, de Mistura said he asked the regime's lead negotiator Bashar al-Jaafari how Damascus defined the term political transition.
"He said, it was... premature to talk about it. My message was premature means imminent as far as we are concerned," de Mistura said.
Assad's fate is by far the most daunting obstacle in the talks aimed at ending the brutal five-year war, which has killed more than 270,000 people and displaced millions.
The main opposition HNC has insisted the president's departure must be part of any peace deal
Speaking to reporters after meeting de Mistura, Jaafari said political transition and Assad's future were "two separate issues."
"President Assad has nothing to do with the... talks," he said, insisting that the subject of the president "is something that is already excluded from the scene".
De Mistura has praised the HNC for submitting "substantive" ideas on a possible transition government, but said the government remained focused on other issues, including negotiating procedures.
While conceding that progress remained slow, de Mistura stressed it was vital opposing sides reach a basic understanding on how to move to a second round, tentatively scheduled for next month.
"I have been reminding everyone that there is no Plan B," the UN envoy told journalists.
He has said that if the talks stall he will turn to "those who have influence" to help clear the roadblocks, listing Assad-ally Russia and the United States, which backs the HNC.
On Monday, de Mistura highlighted the importance of US Secretary of State John Kerry's upcoming meeting in Moscow with his counterpart Sergei Lavrov.
"Nothing is purely coincidental...everything is connected," the UN envoy said of timing of the visit, scheduled to happen just after the talks go on recess.
Negotiations were rattled last week by Russia's surprise decision to withdraw most of its troops from Syria, a move experts said could help the peace drive by weakening Assad's position.
Jaafari insisted Damascus was committed to the peace process, and that his delegation had "clear instructions from our leadership to engage seriously in these talks."
HNC member Yahya Kodmani on Sunday accused the regime of being "obstinate" and suggested Moscow could help move talks forward.
"We hope that Russia will use its powers to pressure the Assad regime in order to move into serious negotiations," he said.
The opposition contingent in Geneva has expanded with the arrival of HNC leader Riad Hijab, a former prime minister under President Bashar al-Assad who defected from his post in 2012.
De Mistura has said the talks had already produced positive outcomes by helping maintain a fragile ceasefire declared on February 27.
Russia on Monday accused Washington of stalling on the enforcement of the truce and not committing to mechanisms for responding to violations.
In a statement, Russian lieutenant general Sergei Rudskoy said that, given the US feet-dragging, Moscow was ready to resort unilaterally to force against ceasefire violators as of Tuesday.
The truce has broadly held since being declared last month and allowed life-saving aid to reach tens of thousands of Syrians stranded in besieged areas.
The ceasefire does not include the Islamic State group (IS) and Al-Qaeda linked Al-Nusra Front, who continue to be targeted in Russian air strikes and government offensives.
At least 26 pro-government fighters were killed battling Islamic State near Palmyra on Monday as Damascus stepped up a bid to recapture the ancient city, the Syrian Observatory for Human Rights said.