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 be 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.

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 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."

British Ambassador Matthew Rycroft said the Russian document was not discussed at Tuesday's council meeting "but we're aware of the Russian proposals."

"We welcome their engagement in the Vienna talks," Rycroft said of Russia. "We welcome any constructive ideas that will accelerate an end of this terrible conflict."

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 and flooded neighboring countries and Europe with more than 4 million refugees.

Russia, Syria's closest ally, has stepped up its diplomatic efforts in the country in recent months, hosting meetings with some opposition groups. It launched a bombing campaign on Sept. 30 which it has claimed is striking terrorists in Syria, but the U.S. says 85 to 90 percent of the strikes have hit moderate Syrian opposition forces and have also killed civilians.

De Mistura said he hopes the second round of talks in Vienna 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 extremist group ISIS, also known as ISIS, 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 ISIS 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."

When the Syrian government and opposition delegations meet under U.N. auspices, the document says they should agree on a series of steps:

- Launching a constitutional reform process taking up to 18 months to guarantee "sustainable security and fair balance of interests, rights and obligations of all ethnic and confessional (religious) groups in structures of power and state institutions."

- Forming a constitutional commission "to embrace the entire spectrum of the Syrian society, including domestic and outside opposition," with the chair agreed by all participants.

- Submitting the draft constitution to a popular referendum, and "after its approval an early presidential election will be called."

- Postponing parliamentary elections planned for spring 2016 and scheduling them to be held simultaneously with presidential elections on the basis of the new constitution. Assad was elected to a new seven-year term in 2014 elections that the opposition dismissed as a sham because of the impact of the civil war.

- Agreeing that the popularly elected president will be commander-in-chief of the armed forces and have "control of special services and foreign policy."

The Russians also propose establishing a Syria Support Group to help prepare the meeting and assist the Syrian parties during negotiations to reach "mutual consent."

It suggests that the group include many of the same members that are taking part in the Vienna talks: the U.S., Russia, China, Britain, France, Saudi Arabia, Turkey, Iran, Egypt, Jordan, Oman, Qatar, United Arab Emirates, Iraq, Lebanon, Germany, Italy, the U.N. special envoy, Arab League, Organization of Islamic Cooperation and the European Union.

Chinese Ambassador Liu Jieyi told reporters, "It is imperative that we start a political process by the Syrian people to work out the future of the country. And the process should be Syrian-led and Syria-owned."

Lithuania's U.N. Ambassador Raimonda Murmokaite said the council "talked a lot about the Vienna momentum, which in a way is what we all need. We need to hold on to a straw, given the level of tragedy. But then again, we need to see how it all proceeds."