WASHINGTON (Reuters) - Two U.S. F-22 fighter aircraft fired warning flares on Wednesday after two Russian Su-25 jets entered an agreed upon deconfliction area in airspace east of the Euphrates river in Syria, the U.S. military said.

Eric Pahon, a Pentagon spokesman, said the F-22 aircraft were providing air cover to partnered ground forces when the Russian jets came into airspace near Albu Kamal. The incident lasted for about 40 minutes and coalition officials contacted the Russians through a communication link to avoid a miscalculation.

At one point, Pahon said, a Russian jet came close enough that one of the F-22 aircraft had to “aggressively maneuver to avoid a midair collision.”

U.S. officials have said that as the battlefield against Islamic State shrinks, they expect Russian and U.S. aircraft to be in closer proximity.

Since early November, Russian jets have flown east of the Euphrates river in the deconflicted airspace about six to eight times a day, according to the U.S. military.

“It’s become increasingly tough for our pilots to discern whether Russian pilots’ actions are deliberate or if these are just honest mistakes,” Pahon said.

“The Coalition’s greatest concern is that we could shoot down a Russian aircraft because its actions are seen as a threat to our air or ground forces,” he added.

U.S. Defense Secretary Jim Mattis has said the U.S. military will fight Islamic State in Syria “as long as they want to fight,” describing a longer-term role for U.S. troops after the insurgents lose all of the territory they control.