MOSCOW—A war of words between the Kremlin and the White House over Syria showed no sign of abating Monday, as Russian Foreign Minister Sergei Lavrov dismissed U.S. evidence of the Assad regime's chemical-weapons use as unconvincing.

Speaking during an appearance at the Moscow State Institute of International Relations, Mr. Lavrov said there is "nothing concrete" in the information the U.S. has shown Russia on the use of such weapons by Syrian leader Bashar al-Assad's regime.

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