PARIS — Russian forces threatened to shoot down a French surveillance aircraft patrolling in international airspace over the Black Sea, a signal of increasingly aggressive behavior from Moscow as its invasion of Ukraine struggles to make headway, French defense officials said Thursday.

"A Russian air traffic control system threatened to shoot down French aircraft in the Black Sea when we were in a free international zone where we patrol,” the French defense minister, Sébastien Lecornu, said on RTL radio.

A French military spokesman, Col. Pierre Gaudillière, said Lecornu was referring to an incident in mid-November that involved one of France’s four giant Airborne Warning and Control System, or AWACS, surveillance aircraft that was flying over international waters in the Black Sea. Gaudillière described the incident as unprecedented for French pilots in that region.

“Through a radio exchange, it was threatened by the Russians," Gaudillière said.

“It was a particularly aggressive radio exchange,” he added. “It’s a first.”

Two other officials said the exchange was in English and that Russian air traffic control said its forces would “destroy" the French aircraft. The officials spoke on condition of anonymity to discuss the classified details.

In his radio interview, Lecornu said Russia is returning to a “particularly aggressive” posture reminiscent of the former Soviet Union's behavior during the Cold War.

“The behavior of Russia in 2024 bears no relation to what we saw in 2022 and, obviously, before the aggression in Ukraine,” the minister said. “It is explained by the fact that Russia is in difficulty on the battlefield in Ukraine."

French air force pilots regularly patrol on the eastern flank of NATO , part of efforts by the 31-nation military alliance to boost its defenses since Russia launched its full-scale invasion of Ukraine two years ago come Saturday.

Aboard a French air force AWACS flight to the Black Sea coast in January, one of the pilots told The Associated Press that they seek to defuse any tensions if they're intercepted by Russian planes, which he said is rare.

“Our orders are to be, let’s say, passive,” the pilot, Major Romain, said. “For a civilian, let’s say ‘polite.’”

Flying high above the Black Sea coast, the French AWACS use their powerful radar and other surveillance gear to peer across to the Crimean Peninsula that was seized from Ukraine by Russia and annexed in 2014. The flights can spot missile launches, airborne bombing runs and other military activity in the Ukraine war.

Russian pilots have at times made clear that they don’t like being watched.

In 2022, a Russian fighter jet released a missile near a British air force RC-135 Rivet Joint surveillance aircraft that was flying in international airspace over the Black Sea, Britain’s government said . The United States government released video in March 2023 of a Russian fighter jet dumping fuel on a U.S. Air Force surveillance drone . The drone crashed into the Black Sea.