Russian Foreign Minister Sergey Lavrov described on Monday as "outrageous" a threat by powerful rebel groups in Syria against those who attend peace talks on the conflict backed by Moscow and Washington.

Russian Foreign Minister Sergey Lavrov described on Monday as "outrageous" a threat by powerful rebel groups in Syria against those who attend peace talks on the conflict backed by Moscow and Washington.

"It is outrageous that some of these extremist, terrorist organizations fighting government forces in Syria are starting to make threats," Lavrov said in televised comments.

"The threats are directed at those who have the courage to attend the proposed Geneva conference being offered by Russia and the United States with the entire world's support," he added.

Nineteen extremist groups fighting to topple Assad issued a statement on Sunday saying the proposed conference "is not, nor will it ever be our people's choice or our revolution's demand."

They went on to say that anyone who attends such talks would be committing "treason" and "would have to answer for it before our courts."

The Geneva conference was proposed jointly by Lavrov and US Secretary of State John Kerry in May but has since been repeatedly delayed.

United Nations Secretary General Ban Ki-moon is still to name a formal date for the proposed meeting despite comments from some Syrian and Arab nation officials suggesting the talks could be held late next month.

Lavrov said the Islamist commanders' threats implied that Russian "diplomatic representative offices and diplomats (working) in foreign countries would be legitimate targets" of assassination and other attacks.

"This is all outrageous and unacceptable. And all of this, in the end, rests on the conscience of those who finance and arm these opposition groups," Russia's top diplomat said.