Moscow's warning came shortly before Russian and Syrian security forces were due to start overseeing the removal of YPG fighters and weapons at least 30 km (19 miles) into Syria, under the deal struck by presidents Vladimir Putin and Tayyip Erdogan.
A complete pullout of the YPG would mark a victory for Erdogan, who launched a cross-border offensive on October 9 to drive the Kurdish militia from the border and create a "safe zone" for the return of Syrian refugees.
The accord, which expands on a U.S.-brokered deal last week, also underlines Putin's dominant influence in Syria and seals the return of his ally President Bashar al-Assad's forces to northeast Syria for the first time in years, by endorsing the deployment of Syrian border guards from noon (0900 GMT) on Wednesday.
Six days later, Russian and Turkish forces will jointly start to patrol a 10 km strip of land in northeast Syria where U.S. troops for years were deployed along with their former Kurdish allies.
Those changes reflect the dizzying pace of changes in Syria since President Donald Trump announced the withdrawal of U.S. forces from northern Syria earlier this month, shaking up the military balance across a quarter of the country after eight years of conflict and prolonged freezes on the frontlines.
Kurdish militia commanders have yet to respond to the deal reached in Russia's Black Sea resort of Sochi, and it was not immediately clear how their withdrawal could be enforced.
A joint Turkish-Russian statement issued after six hours of talks between Putin and Erdogan said they would establish a "joint monitoring and verification mechanism" to oversee implementation of the agreement.
Kremlin spokesman Dmitry Peskov was more blunt. If Kurdish forces did not retreat, Syrian border guards and Russian military police would have to fall back. "And remaining Kurdish formations would then fall under the weight of the Turkish army," he said.
In a swipe at Washington, which has called into question how the deal will be guaranteed, Peskov said the United States had been the closest ally of the Kurdish fighters but had now betrayed them.
"Now they (the Americans) prefer to leave the Kurds at the border and almost force them to fight the Turks," he said in remarks to Russian news agencies.
The Kurdish-led SDF were Washington's main allies in the fight to dismantle Islamic State's self-declared caliphate in Syria. Trump's decision to pull troops out was criticized by U.S. lawmakers, including fellow Republicans, as a betrayal.
In a further sign of growing ties between Ankara and Moscow, which have alarmed the U.S. administration, the head of Russia's defense sales agency was quoted by the Interfax news agency on Wednesday as saying Moscow could deliver more S-400 missile defense systems to Turkey.
Turkey, a NATO member, has already been frozen out of a programme to buy and help produce F-35 jets and faces possible U.S. sanctions for buying the S-400 systems, which Washington says are incompatible with NATO's defenses and threaten the F-35 if operated near the stealth fighter.
Overnight, Turkey's defense ministry said that the United States had told Ankara the YPG had completed its withdrawal from the area of Turkey's military offensive.
There was no need to initiate another operation outside the current area of operation at this stage, the ministry said, effectively ending its military offensive that began two weeks ago, drawing widespread criticism.