A pair of U.S. Air Force stealth fighter jets intercepted two Russian attack jets in Syria, an area off-limits to Russian jets based on a long-standing mutual agreement, a U.S. defense official told Fox News on Thursday.

During the incident, both American F-22 jets fired flares after cutting across the front of the Russian jets, a tactic known as ‘head-butting,’ meant to send a strong warning to an opposing warplane.

Pentagon spokesman Eric Pahon confirmed the 40-minute long incident to Fox News, adding that at one point a Russian attack jet flew close enough to an F-22 that it had to aggressively maneuver to avoid a midair collision.

The incident took place Wednesday, east of the Euphrates River, the official said.

U.S. military officials raised concerns about the incident through a hotline established in the past two years between American and Russian air forces.

In recent months, Russian jets have increasingly flown in restricted areas in eastern Syria, violating long established agreements with the U.S.-led coalition.

The Pentagon spokesman said it has become “increasingly tough” to determine whether Russian pilots’ actions are deliberate or honest mistakes.

"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," Pahon told Fox News.

The Russian Air Force has been supporting the Syrian government since sending dozens of fighter and attack aircraft to an airbase near the Mediterranean Sea in September 2015. In June, a U.S. Navy F-18 Super Hornet shot down a Syrian fighter jet, the first time in 18 years the U.S. military has downed an enemy aircraft in air-to-air combat.

