By John Irish, Stephanie Nebehay and Tom Miles
GENEVA (Reuters) - Syria's warring sides both claimed U.N. peace talks in Geneva were going their way on Thursday, but the chief government negotiator and Syria's ally Russia accused the main opposition delegation of trying to wreck the talks.
Syrian government negotiator Bashar al-Ja'afari told reporters that he had discussed "substantive issues, particularly counter-terrorism" with U.N. mediator Staffan de Mistura, and would continue on Friday.
The government delegation has sought to focus peace talks around counter-terrorism - which is how it frames the battle with all its armed opponents - while the opposition wants to discuss political transition, which it says must mean President Bashar al-Assad giving up power.
Ja'afari said the opposition High Negotiations Committee was holding the peace talks hostage, and accused some of its members of "high treason" by receiving support from Saudi Arabia, Israel and Turkey.
"The Riyadh opposition will be held responsible for any failure of the Geneva talks," Ja'afari said.
Russia is seen as holding the balance of power in Syria and as being the powerbroker behind the peace talks.
On Wednesday, it appeared to offer an olive branch to the HNC, enemies of Moscow's ally Assad, when Deputy Foreign Minister Gennady Gatilov received the opposition leadership at Russia's mission in Geneva.
Russian media quoted Gatilov as saying the meeting had been constructive. But on Thursday, Moscow's tone soured.
"The talks are once again raising questions about the ability of representatives of the Syrian opposition to do a deal," 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 that curry favour in Moscow.
The same line was later repeated by Ja'afari.
The chief opposition negotiator Nasr al-Hariri said he had also held very positive discussions with de Mistura, centred on the opposition's goal of a political transition for Syria.
Like Ja'afari, he also expected another meeting with de Mistura on Friday, which will mark a full week of talks, on top of some preliminary meetings. But there is still no hint of any real breakthrough, and each side has yet to accept the other's demands.
"The prospects are very dim," HNC negotiator Basma Kodmani said.
Russia has sought to revive diplomacy since its air force helped the Syrian army 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 a shaky ceasefire and has tried to expand the scope to cover political aspects such as a discussing a new constitution.
Despite the ceasefire nominally in force since late December violence has escalated since the onset of the talks a week ago and the warring sides appear no closer to actual negotiations.
Zakharova said Russia was determined to press ahead with the separate track of peace talks in Kazakhstan and was ready to include new participants in those talks, including those from the Syrian opposition who would behave constructively.
A new round of Astana talks is due on March 14, although the last round in February failed to yield results.
"Russia sees an opportunity here to manage the political track in a different way," said a Western diplomat.
(Additional reporting by Tom Miles, Stephanie Nebehay in Geneva and Denis Pinchuk in Moscow; Editing by Alison Williams and Dominic Evans)