Russia warned Syria on Tuesday not to use chemical weapons and said it assumed its ally would adhere to its international obligations.
The Russian Foreign Ministry said Syria in 1968 had ratified a 1925 international protocol that bars the use of poison gases as a method of warfare.
"The Russian side proceeds from the assumption that Syrian authorities will continue to strictly adhere to the undertaken international obligations," it said in a statement.
There has been a barrage of warnings about Syria's chemical arsenal this month, especially strident from the United States and Israel, but this is now accompanied by firm but private advice from Russia, Assad's main international ally, to put an end to speculation he might use it.
One Western diplomat in the region said: "There was talk of them using it two weeks ago, but the Russians intervened quickly to stop him.
"If you think how desperate these people are and what they have done in the past, you have to assume they would be prepared to use it. All of us think he (Assad) is capable of using it and will do it if he was pushed to the wall," the diplomat said, referring to credible reports that Assad was preparing to use Sarin gas against Syrian rebels.
But "the Russians got hold of him and told him 'don't even think about it'".
Syrian Foreign Ministry spokesman Jihad Makdissi acknowledged on Monday that the country had chemical weapons, saying it would not use them to crush rebels but could use them against forces from outside the country.
Russia has blunted Western and Arab efforts to push Syrian President Bashar Assad from power but sometimes has criticized his handling of a 16-month uprising in which activists say at least 18,000 people have been killed.
Damascus has not signed a 1992 international convention that bans the use, production or stockpiling of chemical weapons, but officials in the past had denied it had any stockpiles.
The foreign ministry said it "would like to underline that Syria joined"
a Geneva protocol on the non-use of such weapons and "presumes that the
Syrian authorities will continue to rigorously abide by its assumed
The ministry had earlier Tuesday re-issued an earlier Syrian travel advisory warning to all Russian citizens.