The United Nations said on Wednesday no date had been set for the next round of Syria peace talks, contradicting a report quoting Russia's deputy foreign minister as saying talks would resume in Geneva on May 10.
UN Special Envoy Staffan de Mistura is struggling to keep the peace process alive after the main opposition High Negotiations Committee (HNC) left formal talks last week.
Asked on Wednesday whether a new date had been set, the HNC said it was up to the UN to say when peace talks would resume but that it would not take part until its demands were met.
Russian Deputy Foreign Minister Mikhail Bogdanov's comments were reported on Wednesday, but a spokeswoman for de Mistura said in an email that May 10 was speculation.
De Mistura was due to address the UN Security Council by video-link on Wednesday night from Geneva at the end of a two-week round which began on April 13.
He is expected to speak beforehand with both US Secretary of State John Kerry and Russian Foreign Minister Sergei Lavrov, the co-sponsors of the fragile two-month ceasefire who are steering the talks.
De Mistura is talking about May 14-15 for starting the next round, a Western diplomat said.
"But it is very, very theoretical," the diplomat said. "It is not at all a given that the two parties will return to Geneva."
"De Mistura feels that ending the round without giving a date for the next one would not be a good sign. But it's theoretical," he added.
The Syrian regime said on Tuesday it had held a final session of talks with de Mistura in a "useful and constructive round," but diplomats warned that an escalation of fighting around Aleppo threatened the shaky peace process.
George Sabra of the HNC said on Wednesday his group would not attend talks unless the situation on the ground improved.
"All the while real steps aren't taken on the ground in Syria, the participation of the delegation of the HNC will remain suspended," Sabra said.
Russian Foreign Ministry spokeswoman Maria Zakharova said on Wednesday that the HNC's conduct was "worrying" and that the group had failed to make constructive proposals at the talks.
The situation on the ground in Syria and the state of the peace talks are a "cause of serious concern," she told a news briefing.
De Mistura still aims to convene a ministerial meeting of major and regional powers under the International Syria Support Group (ISSG), before the next round is held, the Western diplomat said.
This would probably be next week, although Russia was not yet on board, he said.