Russian President Vladimir Putin has ratified an intergovernmental agreement on the deployment of the Russian Armed Forces' air force in Syria.

The deal stipulates that the Russian air force will be deployed at the request of the Syrian government. Deployment will be at the Hmeymim airfield in western Syria "indefinitely" AND free of charge.

Earlier, a Federation Council official said the deployment served the purpose of defense and was not targeted at a third party. The agreement was signed in Damascus in August, 2015 and approved by both houses of Russia in October.

Russia launched its counter-terrorism air campaign in Syria in September of 2015, at the request of Syrian President Bashar Assad.