The US-led coalition downed a Syrian army jet in the southern Raqqa countryside during a Syrian combat mission against Islamic State militants, the Syrian army says.
The US-led Operation Inherent Resolve Coalition have confirmed the incident.
The Pentagon said the shoot-down came hours after Syrian government-backed forces attacked US-backed fighters, known as the Syrian Democratic Forces, in the village of Ja'Din.
As the attack unfolded, the US military used a deconfliction channel to communicate with the Syrian government to stop the skirmish, according to the Pentagon statement.
Roughly two hours later, despite the called-for stand down, a Syrian Su-22 jet dropped an unknown number of munitions on the Syrian Democratic Forces.
A US F/A-18 promptly shot the Syrian aircraft down "in accordance with rules of engagement and in collective self-defence of coalition partnered forces", the Pentagon statement said.
An army statement released on Syrian state television said the plane crashed and the pilot was missing. It said the incident took place on Sunday afternoon near a village called Ja'Din.
"Aircraft from the 'international coalition' targeted one of our fighter planes in the Resafa region of southern Raqa province this afternoon while it was conducting a mission against the terrorist Islamic State group," the army said in a statement.
The "flagrant attack was an attempt to undermine the efforts of the army as the only effective force capable with its allies ... in fighting terrorism across its territory", the Syrian statement said.
"This comes at a time when the Syrian army and its allies were making clear advances in fighting the Daesh (Islamic State) terrorist group."
But the US-led Coalition says the shoot-down was in clear self-defence, coming after clear warnings were issued that it was US-backed forces that were being bombed.
The US Combined Joint Task Force for Operation Inherent Resolve have released a statement saying the shoot-down was in response to an attack by pro-Syria regime forces attacking the Syrian Democratic Forces town of Ja'Din, south of Tabqah, driving them out of the area.
"Coalition aircraft conducted a show of force and stopped the initial pro-regime advance toward the SDF-controlled area," the statement reads.
The US-led coalition has in recent weeks escalated its own aerial bombing campaign in northern Syria and Raqqa province.
US-backed forces have encircled the city of Raqqa and captured several districts from the militants.
The Syrian army has also taken territory from retreating Islamic State militants in the western Raqqa countryside and seized back some oilfields and villages that had been under the militants' control for almost three years.
Fighting broke out on Sunday for the first time in Syria between regime troops and a US-backed alliance of Kurdish and Arab fighters, a monitoring group said.
The Syrian Observatory for Human Rights said the clashes took place in the evening at two villages some 40km south of the city of Raqa before the Syrian army accused the US-led coalition of downing one of its warplanes.
- Additional reporting by The Washington Post