Territory held by the Syrian Democratic Forces, an alliance of Kurdish and Arab fighters, amounts to around 23 percent of Syria, the group added.
Open Russian military intervention in Syria began in September 2015, with the air force carrying out punishing airstrikes on opposition forces in support of Bashar al-Assad.
The devastating air campaign is believed to be responsible for the deaths of tens of thousands of civilians, most notably during the Aleppo offensive when the rebel-held east of the city was decimated by a Moscow-led blitz.
Country in ruins
While Assad has regained momentum in the conflict with critical backing from Iran, Hizballah and Russia, much of Syria lies in ruins as the result of the war, with 400,000 people killed, public infrastructure destroyed and millions displaced.
The World Bank estimates that the conflict in Syria has cost the economy some $226 billion, while fighting has destroyed 27 percent of Syria's housing stock and about half of Syria's medical and educational facilities.
Around 85 percent of the population lives below the poverty line while half are unemployed.
The central role of foreign states Iran and Russia, together with powerful Shia militia groups such as Hizballah, also undermines Assad's claims to have protected Syria's sovereignty.
Earlier this year, Damascus agreed to lease the Syrian port of Tartus, along with the airbase, to Russia, meaning that Moscow's military can base jet fighters and warships in Syria for decades to come.
Russian military police provide critical manpower on the ground for monitoring checkpoints in the so-called "de-escalation zones" across the country, part of a deal agreed by foreign powers at Astana, while Assad has signed key memorandums with Iran which will benefit Iranian companies in post-conflict reconstruction projects.