The ministry said in a statement that a convoy of Russian military police had crossed the river at noon (0900 GMT) and "advanced towards the Syrian-Turkish border".

The deal, reached between Russia and Turkey on Tuesday, will see Russian military police and Syrian border guards "facilitate the removal" of Kurdish People's Protection Units (YPG) fighters and their weapons from within 30 kilometres (18 miles) of the border.

This withdrawal must be finalised within 150 hours.

The defence ministry published a map showing where 15 Syrian checkpoints would be set up along the border.

News agency RIA Novosti quoted the ministry as saying military police had met with officials from the border town of Kobane and were due to start patrolling there.

The Turkey-Russia agreement was reached after marathon talks between President Recep Tayyip Erdogan and President Vladimir Putin in Russia on Tuesday.

It will also see Turkey preserve a "safe zone" inside Syria about 120 kilometres long (75 miles) and 32 kilometres deep. Russia and Turkey will eventually launch joint patrols along the zone.