Russian warplanes have reportedly bombed the rebel-held Syrian province of Idlib, where troops have been preparing for a major offensive.
If confirmed, they would be the first such air strikes there in three weeks.
Earlier, US President Donald Trump warned Syria's Bashar al-Assad against launching a "reckless attack" on Idlib.
But Kremlin spokesman Dmitry Peskov rejected the warning and said the Syrian army was "getting ready" to clear a "nest of terrorists" there.
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Mr Peskov said the al-Qaeda-linked jihadist fighters who dominate Idlib were threatening Russian military bases in Syria and blocking a political solution to the seven-year civil war.
The UN has warned that an attack on Idlib, where 2.9 million people live, "will overwhelm capacities and has the potential to create a humanitarian emergency at a scale not yet seen through this crisis".
It estimates that as many as 800,000 people could be displaced and that the number of people in need of aid, which is already high, could increase dramatically.