UNITED NATIONS -- Russia has circulated a document on ending the nearly five-year-old Syrian conflict that calls for drafting a new constitution in up to 18 months that would be put to a popular referendum and followed by an early presidential election.

The document, obtained Tuesday by The Associated Press, makes no mention of Syrian President Bashar Assad stepping down during the transition -- a key opposition demand. It only mentions that "the president of Syria will not chair the constitutional commission."

It calls for U.N. special envoy Staffan de Mistura to launch a political process between the Syrian government and "a united delegation of the opposition groups" on the basis of the June 2012 communique agreed upon by major powers in Geneva calling for the establishment of a transitional governing body for Syria with full executive powers, leading to elections.

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Russia's deputy U.N. ambassador Vladimir Safronkov, who expressed regret that the document had been leaked, told The Associated Press: "It's our vision -- it's our proposal."

"And of course we are receptive for proposals from the other side. It's just Russian contribution, how we launch a political process... to make parties work together, government and opposition," he said.

The document was circulated ahead of a second round of talks in Vienna on Saturday among key governments on both sides of the Syrian conflict. De Mistura told reporters after briefing the U.N. Security Council late Tuesday that his message to the 15 members was "one word -- momentum."

At the initial talks in Vienna on Oct. 30, the U.S., Russia, Iran and more than a dozen other nations agreed to launch a new peace effort involving Syria's government and opposition groups. But they carefully avoided the issue of when Assad might leave power -- a dispute at the heart of the conflict that has claimed more than 250,000 lives.

De Mistura said he hopes the second round will "bring some deliverables for the Syrian people, and one of them should be reduction of violence... and I hope something in that direction can be achieved."

The Russian document, entitled "Approach To The Settlement of The Syrian Crisis," focuses on both opposition and "terrorist groups," and the need to differentiate between them.

It calls for the Security Council to agree to list the Islamic State of Iraq and Syria (ISIS, also known as ISIL) as "a terrorist organization," and agreement on an additional list of terrorist groups.

"When considering the issue of a cease-fire in Syria, operations against ISIL and other terrorist groups must be excluded," the document says.

It says the composition of the opposition delegation to the political talks "has to be agreed beforehand, including on the basis of readiness of respective groups to share the goals of preventing terrorists from coming to power in Syria and of ensuring sovereignty, territorial integrity and political independence of Syria, as well as secular and democratic character of the state."