A senior Russian official warned the United States on Saturday against making any "reckless" moves in Syria, the RIA news agency reported.
Reacting to comments by U.S. National Security Adviser John Bolton about Washington's possible strong reaction in the event of a chemical or biological attack in the Syrian region of Idlib, Deputy Foreign Minister Sergei Ryabkov said: "We warn the Americans and their allies against taking new reckless steps in Syria."
"We are hearing ultimatums from Washington ... and it does not affect our determination to continue our policy for the total elimination of terrorist epicentres in Syria and the return of this country to a normal life," RIA quoted Ryabkov as saying.
The Idlib region, a refuge for civilians and rebels displaced from other areas of Syria as well as powerful jihadist forces, was hit by a wave of air strikes and shelling this month, in a possible prelude to a full-scale government offensive.
Russia earlier this week presented proposals to the Turkish authorities on resolving the situation in Idlib, a northwestern Syrian region that borders Turkey.
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Turkey has backed some rebel groups in region and set up a dozen military observation posts. It is trying to avert an attack by forces loyal to Syrian President Bashar al-Assad, who is backed by Moscow.
Syrian Foreign Minister Walid al-Moualem is set to visit Moscow at the end of the month.