A pair of U.S. jets intercepted two Russian fighter aircraft over Syria on Wednesday, the Pentagon said, the kind of highly dangerous yet constant encounter that is occurring with more regularity despite agreements between the countries to avoid potentially deadly mistakes.

Two F-22A Raptors were diverted from supporting ground operations against Islamic State militants and intercepted the Russian Su-25 aircraft after they crossed into U.S. coalition airspace east of the Euphrates River near Bukamal, a key town on the border of Iraq and in the region where militants have congregated following defeat in Raqqa. The Russian warplanes were so close to coalition jets that the jets shot flares and even chaff — clouds of metal meant to confuse radar systems but possibly used in this case as an additional visual warning.

One of the U.S. pilots also performed an aggressive maneuver to avoid a midair collision, said Eric Pahon, a Pentagon spokesman. Multiple calls to the emergency channel established to avoid such issues were made during the 40-minute encounter, Pahon said, culminating in a tense moment when one of the F-22s shadowed its Russian counterpart.

The Syrian skies have become another contested battle space between old adversaries, as President Bashar al-Assad’s forces backed by Russian air power and advisers on the ground have defeated rebel groups. Defense officials worry that continued fights over airspace could quickly heighten tensions between the nations as pilots make split-second decisions at the helm of armed jets flying hundreds of miles an hour, raising the chance of a miscalculation that results in either a collision or a shoot-down if they feel coalition troops are at risk.

Russians agreed in November to maintain flights west of the Euphrates while coalition aircraft would maintain corridors in the east, with the understanding that they would use established deconfliction channels between senior commanders first established in 2015 to avoid catastrophes.

“We’re trying to kill ISIS. But we’re also trying to contend with unsafe interaction with Russian jets,” Pahon said, using an acronym for the Islamic State. Since the agreement, Russians have flown east of the river six to eight times a day, which is about 10 percent of all Russian and Syrian flights, he said.

Moscow’s defense minister disputed U.S. version of events, suggesting in a statement the interaction interrupted protection for a humanitarian aid convoy. The Su-25s then forced at least one F-22 to leave the area, state-owned media Sputnik News reported.

Russian President Vladimir Putin announced Dec. 11 that his forces will start withdrawing from Syria after a two-year military campaign against ISIS. (Reuters)

Agreements to deconflict were reviewed in September after Russia targeted and wounded U.S. proxy forces near Deir al-Zour, a city on the river where Islamic State fighters have converged in recent months.

“It couldn’t be more complex and crowded in that area,” said the chairman of the Joint Chiefs of Staff, Marine Gen. Joseph F. Dunford Jr., describing the Euphrates River valley following the incident. “Deconfliction is more difficult in that area than it was a few months ago.”

The incident came just days after Russian President Vladimir Putin announced during a visit to a base in Syria that he would scale back air operations, prompting skepticism in the Pentagon based on earlier statements of force reductions that never materialized.