The defense ministers from Russia, Iran, and Syria are due to meet in Tehran on June 9 for talks on the Syrian conflict.
Russia's Defense Ministry said the ministers -- Sergei Shoigu, Hossein Dehghan, and Fahd Jassem al-Freij -- will discuss ways to enhance their cooperation in fighting the Islamic State (IS) extremist group and the Al-Qaeda-linked Al-Nusra Front.
Moscow and Tehran are Damascus’s main international backers, providing the regime with military and financial support.
Russia launched its bombing campaign in Syria in September, significantly improving the position of President Bashar al-Assad's regime.
Russian forces are now backing a push by government troops to capture IS-held territory in the Euphrates Valley.
Moscow has also pledged to step up its air strikes in and around the northern city of Aleppo, accusing rebel forces of failing to deliver on a February cease-fire pledge to break ranks with Al-Nusra.