President Vladimir Putin signed a law ratifying a deal with the Syrian regime allowing Russia to keep its air base in Syria for almost half a century, official documents show.

The original deal, signed in Damascus in January, sets out the terms under which Russia can use its Hmeymim air base in Latakia Province which it has used to carry out air strikes against forces opposing Bashar al-Assad.

Putin approved the agreement on Wednesday, after the two chambers of the Russian parliament backed it earlier this month, according to the government's official information portal. He did not give a timeline for Russia's operation in Syria, which he launched in 2015, and has since announced a scale down several times but Russian activities on the ground have apparently not diminished.

The document says Russian forces will be deployed at the Hmeymim base for 49 years with the option of extending that arrangement for 25-year periods. The base has been at the heart of Moscow's military foray since it intervened in the conflict in September 2015, helping turn the tide in favor of Assad, one of Russia's closest Middle East allies.