Syria conflict: Russia 'peace plan' revealed ahead of key summit
A Russian document circulating at the United Nations has proposed a constitutional reform process in Syria, lasting 18 months, to be followed by presidential elections.
The document does not say whether Syrian President Bashar al-Assad should remain in power during that time.
It says certain Syrian opposition groups should take part in key talks on the crisis in Vienna on Saturday.
The Syrian army meanwhile has broken a siege in the north.
Army units fought their way to Kuwairis airbase, east of Aleppo, and eliminated large numbers of Islamic State (IS) militants, reports said.
The facility had been under attack by IS jihadists for nearly two years.
It represents a victory for regime forces which have struggled to advance even since Russia added its firepower to the conflict at the end of September, flying hundreds of sorties in that time.
The BBC's Steve Rosenberg is embedded with Russian forces at their air base near Latakia, sending the following tweets among others on Wednesday:
At least 22 people were killed and many more wounded by rebel shellfire in the Mediterranean city of Latakia on Tuesday, state media and activists say.
Latakia, which lies in the heartland of President Assad's minority Alawite sect, has largely escaped the conflict that has devastated most of Syria and killed more than 250,000 people.
A Russian foreign ministry spokeswoman said that the priority ahead of next Saturday's meeting should be to establish which Syrian opposition groups are to be regarded as partners in the process, and which are "terrorist" and unacceptable.
Analysing Russia's proposal - Jonathan Marcus, BBC diplomatic correspondent
Two fundamental problems are immediately apparent:
First, there is the position of President Assad himself. The leaked document makes no mention of Mr Assad standing down during the transitional process, though it does say: "The president of Syria will not chair the constitutional commission".
The second problem is that of inclusivity - who actually will be asked to participate in the eventual peace talks?
The Russian proposal speaks of the launching of a political process between the Syrian government and "a united delegation of opposition groups".
But what exactly does this mean? Will it include many of the rebel groups backed by Turkey, the Gulf Arab states and the West, some of who have taken the brunt of Russian air strikes? How can these diverse and in many cases highly fragmented and localised groups be pulled together into a delegation that can speak with a common voice?
The eight-point proposal drawn up by Russia is reported not to rule out President Assad's participation in the elections - something his enemies say is impossible if there is to be peace.
"[The] popularly elected president of Syria will have the functions of commander-in-chief of the armed forces, control of special services and foreign policy," the leaked document is quoted by news agencies as saying.
It says that the reform process should not be chaired by President Assad, but by a candidate agreed upon by all sides.
It also calls for UN Special Envoy Staffan de Mistura to launch a political process between the Syrian government and "a united delegation of opposition groups" on the basis of the June 2012 agreement between major powers in Geneva, which calls for the formation of a transitional government for Syria.
Mr de Mistura on Tuesday urged world powers to build on the "momentum" of new international talks to help devise a roadmap to end the four-year war.
"The momentum in Vienna needs to not be missed," he told reporters after briefing the UN Security Council.
About 20 countries and international groups will attend the talks which have as their central objective a ceasefire between President Assad's forces and some opposition groups.
The successful assault to relieve Syrian government forces holed up inside the Kuwairis base by IS comes a week after the Syrian army battled jihadists to regain control of a road south-east of Aleppo and took back control of the government's only supply route into the city.
Areas around Aleppo have seen weeks of heavy fighting after Syrian troops, backed by Lebanese and Iranian fighters, launched an offensive to retake surrounding territory from rebels and jihadist fighters.
The regime offensive has so far focused on clearing rebel-held areas south of Aleppo, rather than targeting the city itself.