Syria's defence minister is to hold Tehran talks with his Iranian and Russian counterparts on Thursday as government forces press an offensive against the Islamic State group, state media said.

Moscow has sent warplanes and special forces in support of President Bashar al-Assad's regime, while Tehran has deployed military advisers and trained and equipped pro-government militias.

The visit by Syria's General Fahd Jassem al-Freij comes as the Damascus regime steps up its campaign against both IS and rebel fighters in second city Aleppo it charges are in cahoots with Al-Qaeda.

Russian Defence Minister Sergei Shoigu travels to Tehran after Moscow pledged to step up its air strikes against rebel forces in and around Aleppo, accusing non-jihadist rebels of failing to deliver on a February ceasefire pledge to break ranks with Al-Qaeda's Syria affiliate Al-Nusra Front.

The talks are being held at the invitation of Iran's General Hossein Dehghan, the state IRNA news agency said.

Tehran has provided Damascus with military and financial support ever since the uprising against Assad's regime erupted more than five years ago.

Moscow's military intervention last September has significantly improved the regime's position.

Russian forces helped government troops to recapture the famed ancient city of Palmyra in March and are now backing a push towards Syria's largest dam at Tabqa in the Euphrates Valley.