Russia has said it will not allow clashes between Turkish and Syrian forces, as Turkey's military offensive in northern Syria continues."This would simply be unacceptable... and therefore we will not allow it, of course," said Moscow's special envoy for Syria, Alexander Lavrentyev, reports BBC.
The withdrawal of US troops from the region, announced last week, gave Turkey a "green light", critics say.
Russia is a key military ally of Syrian leader Bashar al-Assad.
Russia's defence ministry said its forces, which have been deployed in Syria since 2015, were patrolling along the "line of contact" between Syrian and Turkish forces.
Kurdish-led forces have been a key ally of the US in the fight against the Islamic State (IS) group in Syria. They described the US withdrawal, which preceded Turkish action, as a "stab in the back".
Turkish President Recep Tayyip Erdogan insists the operation will continue until his country's "objectives have been achieved" despite the involvement of Syrian government forces.The US has imposed sanctions on Turkish ministries and senior government officials in response to the country's military offensive in northern Syria.
US President Donald Trump also phoned his Turkish counterpart to demand an immediate truce, US Vice-President Mike Pence said.
Mr Pence said he would travel to the region "as quickly as possible".