Russian Prime Minister Dmitry Medvedev has warned that sending more foreign ground troops into Syria will result in "a full-fledged, long war".
He hit back at the US Secretary of State John Kerry, who has suggested more countries could enter the conflict if diplomatic efforts fail to reach a solution.
The Russian PM has also said ongoing tensions between Russia and the West have plunged the world into "a new period of Cold War".
Saudi Arabia's foreign minister has insisted Bashar al-Assad will not be ruling Syria in the future, and that Russian intervention in Assad's favour is in vain.
Adel al-Jubeir's comments come in an interview with the German newspaper newspaper Sueddeutsche Zeitung, published on Saturday.
"It might take three months, it might take six months or three years - but he [Assad] will no longer carry responsibility for Syria. Period," al-Jubeir said.
Saudi Arabia has long said that the Syrian president must step down if Syria's conflict is to end.
Syrian President Bashar al-Assad has vowed to regain control of the entire country but warned it could "take a long time".
Marathon talks agree a partial ceasefire but fail to secure an end to Russian bombing, while rapid humanitarian aid won't reach all Syrians.