An agreement between Syrian opposition groups in Riyadh this week excluded important groups like Syrian Kurds and should not form the basis for peace talks, the Russian Foreign Ministry said.

"We cannot accept the attempt by the group which met in Riyadh to assign itself the right to speak on behalf of the entire Syrian opposition," the ministry said in a statement December 12.

Saudi Arabia's talks between Syrian opposition groups excluded Kurds from the outset, despite their holding large tracts of territory in Syria. They also specifically excluded the Islamic militant groups Islamic State and Nusra Front, Al-Qaeda's Syrian affliate, in a move that met Russia's approval.

But most of Syria's other disparate opposition groups signed onto the agreement, which called for elections leading to an all-inclusive, democratic government for Syria, and had been hailed as "an important step" by the United States.

The groups that signed the accord agreed to a cease-fire and said they were prepared to meet with Syrian President Bashar al-Assad in January to start negotiations on a transition of power.

But they riled Moscow by calling for Assad to step down at the start of a transitional period leading to elections.

Moscow said there should be no "preconditions" set for a political transition.

Peace negotiators in Vienna already agreed that "only the Syrian people can decide the fate of Syria," the ministry said. "Agreements must be respected."

Russia also criticized the Riyadh accord for not including members of the so-called "patriotic Syrian opposition" seen as friendly towards Assad.

The foreign ministry said the exclusion of Syria's Kurdish Democratic Union Party, in a major omission, was the result of objections from Turkey, which had a say in who got invited to the Riyadh conference.

Despite excluding the main Islamic militant groups in Syria, as Russia wished, Moscow objected to the inclusion of groups it views as closely related to such "infamous" terrorist groups, including Saudi-backed Jaish al-Islam (Army of Islam), and Ahrar al-Sham.

Moscow says Ahrar al-Sham is an ally of Al-Qaeda and is responsible for regularly shelling its embassy in Damascus.

"We are still convinced that terrorists of all stripes should be excluded from the political process in Syria," the foreign ministry said.

Russia took the position that it is up to UN Syrian envoy Staffan de Mistura to determine which groups will represent the opposition in the end, not Riyadh.

Russia's strong objections to the Riyadh accord presents a challenge for U.S. Secretary of State John Kerry, who travels to Moscow on December 15 to try to further the Syrian peace effort.

Kerry had welcomed the accord as an "important step," although he said it had some "kinks" that needed working out and "difficult work" remained to forge a peace deal.

With reporting by Reuters, AFP, Interfax, and TASS