This just in from Jerusalem Post:  

US warplane shot down a Syrian army jet on Sunday in the southern Raqqa countryside with Washington saying the jet had dropped bombs near US backed forces and Damascus saying the plane was downed while flying a mission against Islamic State militants.

A Syrian 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 Rasafah.

U.S. Central Command issued a statement saying the Syrian plane was downed “in collective self-defense of Coalition-partnered forces,” identified as fighters of the Syrian Democratic Forces (SDF) near Tabqah.

It said that “pro-Syrian regime forces” had earlier attacked an SDF held town south of Tabqa and wounded a number of fighters and driving them from the town.

Coalition aircraft in a show of force stopped the initial advance. When a Syrian army SU-22 jet later dropped bombs near the US backed forces, it was immediately shot by a US F/A-18E Super Hornet, the statement said.

Before it downed the plane, the coalition had “contacted the its Russian counterparts by telephone via an established “de-confliction line” to de-escalate the situation and stop the firing.”
The coalition does “not seek to fight the Syrian regime, Russian or pro-regime forces” but would not “hesitate to defend itself or its “partnered forces from any threat,” the statement said.

The US-led coalition, which has in recent weeks escalated its 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 oil fields and villages that had been under the militants’ control for almost three years.

U.S. shot down a Syrian aircraft after a day of fighting on the ground in an area near Raqqa.  Syrian regime forces attacked U.S. backed fighters and had driven them from a key town. A lot of fighting on the ground between pro-regime forces and the U.S. backed forces.  U.S. came in and conducted a show of force with fighter jets, trying to scare off the pro-regime advances. Later a Syrian SU22 came back in and dropped bombs near the U.S. backed fighters and the U.S.F18 came in and shot down that Syrian jet as part of a defense mission to protect the U.S. backed fighters on the ground, all of this in a critical location essentially ISIS last stand.  

CNN states the following:

The Syrian Armed Forces claims that their planes were attacked “while it was carrying out a combatant mission against ISIS terrorist organization.” The general command called the action a: “flagrant aggression” that affirmed the United States’ “real stance in support of terrorism,” according to Syrian Armed Forces.“The attack stresses coordination between the US and ISIS, and it reveals the evil intentions of the US in administrating terrorism and investing it to pass the US-Zionist project in the region.”