WASHINGTON — The Russian military has been jamming some U.S. military drones operating in the skies over Syria, seriously affecting American military operations, according to four U.S. officials.

The Russians began jamming some smaller U.S. drones several weeks ago, the officials said, after a series of suspected chemical weapons attacks on civilians in rebel-held eastern Ghouta. The Russian military was concerned the U.S. military would retaliate for the attacks and began jamming the GPS systems of drones operating in the area, the officials explained.

Sen. Ben Sasse, R-Neb., reacted to the news of Russian scrambling Tuesday by saying "Russia wants to undermine our interests at every turn."

"It is insane to think that Russia is anything but an adversary," said Sasse.

Jamming, which means blocking or scrambling a drone's reception of a signal from a GPS satellite, can be uncomplicated, according to Dr. Todd Humphreys, the director of the Radio navigation Laboratory at the University of Texas at Austin.

"GPS receivers in most drones can be fairly easily jammed," he said.

Humphreys, an expert on the spoofing and jamming of GPS, warns this could have a significant impact on U.S. drones, causing them to malfunction or even crash. "At the very least it could cause some serious confusion" for the drone operator on the ground if the drone reports an incorrect position or is lost, he said.

U.S. analysts first caught the Russian military jamming drones in eastern Ukraine four years ago, after the invasion of Crimea, according to Humphreys. He said the jammers were initially detected as faint signals from space, bouncing off the earth's surface. The jammers "had a pretty significant impact" on the United Nations surveillance drones that were attempting to monitor the area, grounding the fleet for days and halting intelligence gathering from the air.

The Defense Department will not say whether the jamming is causing drones to crash, citing operational security. "The U.S. military maintains sufficient countermeasures and protections to ensure the safety of our manned and unmanned aircraft, our forces and the missions they support," said Pentagon spokesman Eric Pahon.

But one official confirmed the tactic is having an operational impact on U.S. military operations in Syria.

The officials said the equipment being used was developed by the Russian military and is very sophisticated, proving effective even against some encrypted signals and anti-jamming receivers. The drones impacted so far are smaller surveillance aircraft, as opposed to the larger Predators and Reapers that often operate in combat environments and can be armed.

Dr. Humphreys says that though the attacks occur in cyberspace, the results are still serious.

"They are a little less hostile looking than a kinetic bullet but sometimes the effect can be just as damaging," he said. "It's like shooting at them with radio waves instead of bullets."