Syrian President Bashar al-Assad has requested military assistance from Russia in a letter to President Vladimir Putin, Damascus confirmed on Wednesday.
"The Russian air force was dispatched to Syria after a request from the Syrian state via a letter by President Assad to President Putin that includes an invitation to send the Russian air force as part of President Putin's initiative to fight terrorism," the Syrian leader's office said in a statement.
The statement came after Russia's parliament approved a request from Putin to begin air strikes in Syria.
Moscow gave no details of the planned operation, but said it would be limited in time and would not include ground troops.
The Kremlin's chief of staff, Sergei Ivanov, said Assad had "appealed to the leadership of our country with a request for military aid".
In Damascus, a military source welcomed the development as "positive", saying it was the culmination of long-standing coordination between Syria and Russia.
The move "is something positive and natural between allied states and a supportive friend," the source said.
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"The coordination (with Russia) will now take on other dimensions, beyond what was already the case," he added.
"The areas of coordination will be expanded to includes areas like reconnaissance, information gathering and target guidance, based on the needs on the ground."
For weeks, US officials and other sources have reported a Russian military buildup in Syria, mostly in the coastal Latakia province that is a stronghold of Assad's regime.
Moscow, which already maintains a naval base in Syria's Tartus province, is said to have sent at least 500 troops as well as a slew of military hardware into Syria.
Russia has been a staunch ally of Assad throughout the uprising against him that began in March 2011, providing his regime with weapons and financial assistance.
It has couched its stepped-up help as part of an effort to combat the Islamic State jihadist group, which controls large swathes of territory in Syria and Iraq.
A US-led coalition is already fighting the group in both countries with air strikes, but it does not coordinate with Syria's government.