The officials were not able to confirm whether the drones were armed, or how many missions have been flown to date. The development was first reported by Reuters.

The introduction of drones follows a buildup of Russian military equipment and personnel into Syria.

To date, Russia has over 25 fighter and attack aircraft, 15 helicopters, nine tanks, three surface-to-air missile systems and at least 500 personnel on the ground in Syria, a U.S. official told CNN.

Defense Secretary Ashton Carter spoke with Russian Defense Minister Sergei Shoigu on Friday regarding the escalating Russian posture.

While there has been no follow-up contact between the two since Friday, defense officials are watching the situation closely. The U.S. opposes Russia's policy of bolstering Syrian President Bashar al-Assad, whom the U.S. has stated must leave office for the civil war to be resolved.

"We are keenly aware of what is happening there," Pentagon spokesman Capt. Jeff Davis told reporters Monday. "We continue to believe that anything that's done in support of the Assad regime, particularly militarily, is counter-productive and risks worsening an already bad situation."

READ: Russia sends fighter jets to Syria after talks with U.S.

Pentagon officials are also concerned about any potential run-in with Russian aircraft in Syrian airspace while the U.S.-led coalition continues its military campaign against ISIS. In addition, the U.S. is supporting rebels in fighting the terror group, also known as Islamic State.

While Russia has not flown any combat missions using jets or helicopters in Syria yet, officials said that they are keeping a close eye on all Russian military moves.

"This has the Secretary's attention and it has the attention of our senior leaders, and we're absolutely concerned about the potential for confliction and the need to de-conflict," Davis said.

For its part, the Russian Foreign Ministry used harsh language against anti-Assad forces after announcing that the Russian Embassy in Damascus had been hit by a mortar shell on Sunday.

The Foreign Ministry statement characterized the attack as a terrorist act by Syrian rebels.

"We expect a clear position in regard of this terrorist act from all members of the international community, including regional parties," the Russian Foreign Ministry said in a statement Monday. "This requires not only words, but also concrete actions."

The statement continued, "a mortar attack on the Russian Embassy was carried from the district of Dzhobar where the anti-government militants are. They did not identify themselves as 'Islamic state,' but have external sponsors, who are responsible for influencing the illegal armed groups."