Senior commander Sergei Rudskoi said Russian forces had set up checkpoints and observation posts around a zone in the south-west and in another one covering Eastern Ghouta, near Damascus.
The two areas are part of a broader Moscow-backed plan to create four "de-escalation zones" in rebel-held parts of Syria.
The announcement marks the first deployment of foreign troops to bolster the safe zones as Moscow seeks to pacify Syria after its military intervention swung the six-year conflict in favour of President Bashar al-Assad.
Earlier this month Russia, the United States and Jordan struck a deal to fix the boundaries of this zone and impose a ceasefire in the area.
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Moscow has also informed Israel of its deployment and that the nearest Russian position is 13 kilometres from the demarcation line between Israeli and Syrian troops in the Golan Heights, Rudskoi said.
Under a second deal Moscow said it struck with "moderate" rebels over the weekend in Egypt, Russian forces on Monday also set up two checkpoints and four observation posts in the area covering conflict-ravaged Eastern Ghouta, he added.
Assad's forces have surrounded the Eastern Ghouta region for more than four years, and regime forces have regularly targeted the area.
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