Syria opposition deal under fire ahead of US-Russia meet

Al-Nusra Front, Syria's al-Qaeda affiliate, has rejected a decision by opposition groups to negotiate with President Bashar al-Assad.

Russia also denounced the opposition gathering in Saudi Arabia where a negotiating team was selected as 'unrepresentative'.

One of the key demands agreed by the groups was that Assad and his associates must step down before a transition period to democracy takes place.

The latest developments come as US Secretary of State John Kerry announced he will visit Moscow on Tuesday.

This is part of efforts between the US and Russia - one of Assad's key international backers - to find a peace deal that will end Syria's four year war.

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.

The meeting between the US and Russian diplomats would build on the momentum of Vienna talks, which took place last month.

Nusra leader: We want Sharia We wont break allegiance w/ AlQaeda There's no FSA Political deals abroad cant be implemented on ground @akhbar — Jenan Moussa (@jenanmoussa) December 12, 2015 ">

Talks which took place between opposition groups and rebel military outfits this week in the Saudi capital yielded an agreement to negotiate with Assad's regime.

Al-Nusra Front chief Abu Mohamed al-Jolani rejected the outcome of the talks as a "plot" and accused rebels who had attended the talks - including key military allies in Syria - of "treason".

"It is a plot, not a conference. Such... gatherings must be foiled," said 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".

He also said the moderate rebel military alliance, the Free Syrian Army, does not exist.

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.

Russia's stance is likely to make Kerry's visit difficult, 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.