A U.S. official told CNN Thursday that Russian fighter jets turned off their transponders as they flew into Syria in an apparent attempt to avoid detection. The official said the fighters flew very close to a transport plane that had its transponder on and functioning.

U.S. satellites rapidly saw that the aircraft were there, according to the official.

The assessment over the weekend was that the fighter jets were on their way. The same official said the Russians have begun flying drones around the coastal city of Latakia.

With no ISIS fighters in the area, the move raises serious questions about the Russians' intentions with their military buildup, which the U.S. has questioned the purpose of and watched with wariness. The action points to a higher likelihood that the Russian plan is to prop up Syrian President Bashar al-Assad rather than fight the terror group.

The U.S. has its own effort underway to defeat ISIS but has also said that Assad must go.

Asked about what the U.S. can do about the situation, Defense Secretary Ashton Carter told CNN at a press conference Thursday that "it's a matter of seeing what the Russians do."

Carter said he hopes the Russians will fight ISIS, "but if it's a matter of pouring gasoline on the civil war in Syria, that is certainly not productive from our point of view."

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