"The Syrian and Russian parties agreed in a telephone call between Presidents Bashar al-Assad and (Russia's) Vladimir Putin to reduce the number of Russian air force personnel in Syria," the president's office said on its Facebook page.
"The Russian side said it will continue to support Syria in its battle against terrorism," it said.
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The presidency said the Russian announcement follows "the success registered by the Syrian army with the cooperation of the Russian air force and with the restoration of security in several regions of Syria".
Putin earlier ordered the Russian defence ministry to begin the troop withdrawal from Tuesday.
"The task that was set before our defence ministry and armed forces has as a whole been completed and so I order the defence ministry to from tomorrow start the withdrawal of the main part of our military contingents" from Syria, Putin said.
The Kremlin said Putin called Assad to inform Moscow's long-standing ally of the move that appears to end the main part of its intervention in Syria's conflict that began in September.