[BBC] 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.
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.
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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.
Later on Tuesday, the Pentagon said F-15 fighter jets and Apache helicopter gunships had been deployed in a show of force towards Turkish-backed forces who had come close to US ground troops near the Syrian town of Ain Issa.
The Turkish-backed fighters had had violated an agreement not to threaten US troops, a military official said.