Russia's Defence Minister Sergei Shoigu on Sunday started a previously unannounced visit to Tehran with the conflict in Syria high on his agenda, Iranian state media reported.

Shoigu met President Hassan Rouhani to deliver a "special message" from his Russian counterpart Vladimir Putin, the official IRNA news agency cited the presidency as saying.

The minister presented an overview "of the situation regarding stability in the region and the process of negotiations for a ceasefire in Syria", where both Moscow and Tehran back the regime, IRNA said.

Rouhani said: "Resolving the crisis in Syria can only be achieved through political negotiations and respect for the rights of the Syrian people who must ultimately decide their country's future."

Shoigu also met his counterpart General Hossein Dehghan after the latter visited Moscow on Tuesday, when Shoigu said Russia and Iran were ready to step up their military cooperation.

Russia and Iran are both longtime allies of Syrian President Bashar al-Assad's regime.

Moscow launched air strikes in support of Syria's government on September 30.

Tehran supports Assad by sending "military advisers" and volunteers to fight alongside the Syrian army.

Russia has also authorised the delivery of S-300 anti-aircraft missile batteries to Iran.

According to Russian and Iranian media, Iran also wants to buy Sukhoi Su-30 fighter jets from Russia.