U.S. officials said the Russian pilots turned off their planes’ transponders to avoid detection, and hid by flying in close formation with Russian AN-124 Condor cargo flights, which used commercial air corridors over Iraq and Iran.

Russian-made surveillance drones began flying over embattled parts of northwest Syria this week to help identify potential targets, U.S. officials said.

The drones fly daily missions above Latakia and Hamah provinces, which are nominally held by Assad’s forces, and Idlib province, which is largely controlled by rebel factions.

Islamic State is not active in any of the areas, however, raising questions about Russian President Vladimir Putin’s intentions in the multi-sided Syrian civil war.