Meanwhile, U.S. Navy Captain Jeff Davis told reporters a U.S. aircraft flying over Syria had to be rerouted to avoid a Russian fighter jet at least once.

"We have taken action to maintain safe separation," Davis said, adding that the U.S. aircraft "changed path a little bit." He did not disclose which type of U.S. aircraft was involved. 

U.S. officials tell Fox News the drone encounters took place over ISIS-controlled Syria, including its de facto headquarters in Raqqa, as well as along the Turkish-Syrian border near Korbani. Another occurred in the northwest, near the highly contested city of Aleppo.

"The first time it happened, we thought the Russians got lucky. Then it happened two more times," said one official. 

The U.S. military's MQ-1 Predator drone is not a stealth aircraft.

"It is easy to see a predator on radar," said one official.