VIENNA (AFP) - Syria's main opposition group on Saturday (Jan 27) said it would boycott Russian peace talks next week in a major blow to Moscow's diplomatic efforts towards resolving the brutal conflict.

The Syrian Negotiation Commission (SNC) accused President Bashar al-Assad's regime and its Russian backers of continuing to rely on military might - and showing no willingness to enter into honest negotiations - as the war inched towards its seventh anniversary.

The announcement came after two days of separate UN-backed peace talks came to a close in Vienna, with an end to the war that has killed more than 340,000 people appearing further than ever.

"This round of negotiations was an international test for the regime, and the test ended yesterday (Friday)," the opposition Syrian Negotiation Commission's Nasr al-Hariri told reporters in Vienna Saturday.

In the SNC's initial announcement of a boycott, the group said on its Twitter account Friday night that "Russia has not succeeded in promoting its conference".

"The SNC has decided not to participate at Sochi after marathon negotiations with the UN and representatives of countries involved in Syria," it added.

Dozens of rebel groups had already refused to join the talks in the Black Sea resort next Monday and Tuesday organised by Moscow, and the question of whether the main opposition would attend overshadowed the Vienna talks.

Those talks stretched late into Friday night, with both regime officials and the SNC meeting separately with UN Syria envoy Staffan de Mistura - who did not strike an especially optimistic tone after the gruelling negotiations.

As with eight previous rounds of failed UN-backed talks in Geneva, there was no sign that the warring sides had met face to face at discussions intended to lay the groundwork for a new post-war constitution.

De Mistura, speaking to reporters early Saturday, admitted there had been a disheartening lack of progress up until now.

"I share the immense frustration of millions of Syrians inside and outside the country at the lack of a political settlement to date," he said.

Russian ambitions

UN Secretary-General Antonio Guterres will send his Syria peace negotiator to the conference in Russia next week, a spokesman said in New York Saturday, despite the Syrian opposition's boycott of the meeting.

De Mistura stressed the legitimacy of the UN-led talks over Russia's parallel peace push, however, saying firmly that a political transition for Syria "is to be reached in the UN-led Geneva process".

"I hope that the forthcoming Syrian national dialogue congress in Sochi will contribute to a revived and credible intra-Syrian process under the UN in Geneva," he added.

Ahead of an SNC press conference on Saturday morning there was little detail about why the opposition had ultimately decided to boycott Sochi, though spokesman Yahya al-Aridi earlier described the talks in Vienna as "tough".

Western powers have viewed the Russian peace initiative - which is also backed by Turkey and Iran - with suspicion, worrying that Moscow is seeking to undermine the UN-backed talks with an ultimate view to carving out a settlement that strengthens its ally, President Bashar al-Assad.

Black comedy

Haid Haid, a consulting research fellow at Chatham House think-tank, said Russia's long-term strategic interests were at play in Sochi.

"They want to present themselves as peace brokers, not only in Syria but in the Middle East in general, a role traditionally carried out by the Americans," Haid told AFP.

"For the Russians to take this role, they have to do what the Americans were not able to do" - find a solution in Syria, he said.

The Vienna talks were also marked by anger from the regime over a leaked set of political proposals from the United States, Saudi Arabia, Jordan, Britain and France that would involve strengthening the role of Syria's prime minister - at the expense of Assad's authority.

Top government negotiator Bashar al-Jaafari told reporters it was "tantamount to a black comedy" that these countries were seeking to shape Syria's political future, as Arabic and English versions of the document circulated online.

"All of them have participated in the bloodshed of the Syrian people," he said of the five nations, blasting the US as the country "that created ISIS" and adding that Saudi Arabia was anything but a "beacon of freedom in the east".