Syria's Al-Qaeda affiliate rejected the outcome of an opposition summit that agreed to negotiate with President Bashar al-Assad, and Russia denounced the gathering in Saudi Arabia as unrepresentative.
The latest developments come with US Secretary of State John Kerry due in Moscow Tuesday in a new bid to push for a peace plan to end Syria's nearly five-year conflict.
In a sign of the tough task ahead, at least 16 people were killed in the Syrian city of Homs Saturday in an attack claimed by the Islamic State group (IS).
The Moscow meeting would build on the momentum of Vienna talks last month and an unprecedented two-day gathering this week in Riyadh among Syria's main political and armed opposition groups.
Those talks yielded an agreement to negotiate with Assad's regime, but also insisted that the president step down at the start of any political transition.
But Al-Nusra Front chief Abu Mohamed al-Jolani rejected the outcome of the talks as a "plot" and accused rebels who had attended of "treason".
"It is a plot, not a conference. Such... gatherings must be foiled," said Jolani, the head of the Al-Qaeda affiliate in Syria, in an interview with Dubai-based Syrian opposition television Orient News.
Jolani said the opposition had met under international "pressure" but warned that even if they reached agreement "they do not have the power to implement it on the ground".
- Russia denounces talks -
Russia, a key Assad ally, also denounced the Saudi talks.
"We cannot agree with an attempt made by the group that gathered in Riyadh to monopolise the right to speak on behalf of the entire Syrian opposition," the foreign ministry said.
It did not say whether a meeting of the 17 nations of the International Syrian Support group would proceed as planned in New York on Friday.
That meeting, under UN auspices, was intended to follow up on Vienna and push forward plans for a negotiated ceasefire in the Syrian conflict.
Moscow also took issue with the exclusion of Kurdish factions from the Saudi talks and the absence of the so-called "patriotic Syrian opposition" tolerated by the Damascus regime.
Signup to our newsletter and follow us on Facebook and Twitter!
Russia's stance is likely to make Kerry's visit tougher, especially after he acknowledged there was "difficult work ahead" as he welcomed the Saudi talks.
In a sign of the complexity of the US-Russian relationship, the State Department said Kerry would meet President Vladimir Putin but the Kremlin would not confirm this.
The Riyadh meeting came after top diplomats from countries that back and oppose Assad's regime agreed last month on a roadmap toward ending the Syrian conflict.
This would see a transitional government set up within six months and elections within 18 months, and calls for negotiations opposition-regime talks by January 1.
Al-Nusra Front ally Ahrar al-Sham attended the beginning of the Saudi talks on Wednesday but later announced it had withdrawn.
Sources inside the talks and Western diplomats said Ahrar al-Sham subsequently signed up to the opposition agreement, but this could not be immediately confirmed.
- Air campaign -
Syria's complex conflict erupted in March 2011 with anti-government protests but billowed into an all-out war after a crackdown by the authorities.
It has seen the rise of so-called moderate rebels but also jihadists including IS, which holds swathes of Syria and neighbouring Iraq and has set up a self-declared caliphate.
Russia began an air campaign in September at the request of the Damascus regime, targeting Assad's opponents.
The Russian raids came a year after a US-led coalition started an air war on the Islamic State organisation, and have expanded since last month's attacks in Paris that killed 130 people and were claimed by the extremist group.
According to the Syrian Observatory for Human Rights, 10 people, including four children, were killed in air strikes believed to have been carried out by Russia on the IS-controlled town of Manbij.
A further 11 people, most of them from the same family, were also killed in air strikes on a village in the eastern province of Deir Ezzor. The Britain-based Observatory could not confirm who carried out those strikes, however. The province and most of its capital is held by IS.
On the ground, the Islamic State group said it was behind an attack on Saturday near a hospital in an Alawite area of the central city of Homs in which at least 16 people died.