Russia's Defense Minister, Sergei Shoigu, on Sunday started a previously unannounced visit to Tehran, AFP reported, citing Iranian state media.
According to the report, Shoigu met President Hassan Rouhani and delivered a "special message" from his Russian counterpart Vladimir Putin.
No further details as to the contents of that “special message” were provided.
The minister also 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, the Iranian news agency 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, noted AFP.
The visit comes amid continued reports that that Iran is on the verge of signing a massive arms deal with Russia and that it would receive advanced S-300 missiles systems from Russia shortly.
Iran said last week that a shipment of the Russian S-300 missile defense systems is currently on its way to Iran and could be delivered this week.
Russia later denied this, saying Iran has yet to pay for the shipment.
The sale of the S-300, originally conducted in 2007, has been repeatedly delayed due to Western pressure given that UN nuclear sanctions ban the delivery to Iran.
But last April, Russian President Vladimir Putin signed a decree lifting a ban on the delivery of the S-300 systems to Iran, explaining that his decision was motivated by Iran's drive to find a solution in talks over its nuclear program, which led to a controversial nuclear deal last July.
In addition to the military deals between the two countries, it was also reported several times that Iranian general Qassem Soleimani visited Moscow at least twice.
Soleimani was targeted by a 2007 United Nations travel ban because of their alleged links to Iran’s nuclear or ballistic missile programs. Russia denied both times that he had visited Moscow.