The Islamic State militant group has urged Muslims to launch a "holy war" against Russians and Americans.
"Islamic youth everywhere, ignite jihad against the Russians and the Americans in their crusaders' war against Muslims," said Islamic State spokesman Abu Mohammad al-Adnani in an audio message released on October 13.
The United States and Russia are conducting separate air campaigns in Syria, which they say are targeting Islamic State.
But Washington says Moscow's campaign has mainly targeted other insurgent groups including those that have fought Islamic State. Russia has denied the allegation.
Russia's two-week-old air campaign has bolstered Syrian President Bashar al-Assad and left the United States struggling to adjust its military support program for Assad's opponents.
The United States and Russia are due to hold another round of military talks on October 14 to avoid an inadvertent clash during their bombing campaigns over Syria.