This is the latest example of tension between the Russian-backed Syrian regime and U.S.-led coalition forces, who are partnering with Arab and Kurdish forces to destroy ISIS.

“The Coalition’s mission is to defeat ISIS in Iraq and Syria,” the U.S. task force in charge of operations in Syria and Iraq announced on Sunday. “The Coalition does not seek to fight Syrian regime, Russian, or pro-regime forces partnered with them, but will not hesitate to defend Coalition or partner forces from any threat.”

Sunday’s incident came after Syrian aircraft attacked Syrian Democratic Forces earlier in the day, wounding several of the fighters, who are allies of the U.S.-led coalition to destroy ISIS, Combined Joint Task Force -Operation Inherent Resolve announced in a news release.

Coalition aircraft conducted a “show of force” mission to stop Syrian forces from advancing and coalition officials contacted the Russians on the “de-confliction line’ to calm the situation down, but nearly two hours later a Syrian SU-22 dropped bombs on SDF fighters, according to Sunday’s news release.

“In accordance with rules of engagement and in collective self-defense of Coalition partnered forces, [the Syrian SU-22] was immediately shot down by a U.S. F/A-18E Super Hornet,” the news release said. “The demonstrated hostile intent and actions of pro-regime forces toward Coalition and partner forces in Syria conducting legitimate counter-ISIS operations will not be tolerated.”

The last time a U.S. pilot shot down a manned enemy aircraft was on May 4, 1999, when Lt. Col. Michael H. Geczy of the 78th Expeditionary Fighter Squadron downed a Serbian Mig-29 over Bosnia using an AIM-120 Advanced Medium-Range Air-to-Air Missile.

More recently, a U.S. aircraft shot down an armed Syrian drone on June 8 after it attacked coalition forces on a joint patrol with partner forces in southern Syria.

The Syrian military claims that the jet shot down on Sunday was flying a mission against ISIS fighters. The pilot was still missing, as of deadline.