"Syria's unity is a red line. This issue is non-negotiable and the idea of federalism is the prelude to the partitioning of Syria," the head of the High Negotiations Committee, Riad Hijab, said in a conference call with reporters.

"This is completely unacceptable," he said. "We agreed on the idea of administrative decentralisation" at the Riyadh conference in December 2015 when the HNC was formed.

On February 29, Russia's deputy foreign minister Sergei Ryabkov said he could not rule out establishing a federal system in Syria if that could help safeguard its unity.

"I cannot assess how likely it is that a federal state would be created in Syria, because the process of defining Syria's future has not yet begun," he said in comments published by Russian news agency Ria Novosti.

"But if after negotiations and consultations on the future of Syria, the (warring) sides... come to the conclusion that this model suits them and that it can safeguard the unity of Syria as a secular, independent and sovereign state, who would object to that?"

Syria's Kurds, who have set up three autonomous regions since the outbreak of the war in 2011, back the idea of federalism.

In January 2012, several Syrian Kurdish groups proposed that a referendum be held to give voters the choice between decentralisation, autonomy or federalism.