A U.S. Navy F-18 flying off the USS George H.W. Bush shot down a Russian-made Syrian SU-22 over the skies of Raqqa after President Bashar Assad's air force attacked U.S.-backed forces attacking ISIS.

The engagement was the first between U.S. warplanes and Syria's air force.

Reuters:

A U.S. warplane shot down a Syrian army jet on Sunday in the southern Raqqa countryside, with Washington saying the jet had dropped bombs near U.S.-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 in the first such downing of a Syrian jet by the United States since the start of the conflict in 2011.

The army statement said it took place on Sunday afternoon near a village called Rasafah.

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 army 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," it added.

The U.S. Central Command later 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, 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 U.S.-backed forces, it was immediately shot down by a U.S. F/A-18E Super Hornet, the statement said.

Before it downed the plane, the coalition had "contacted 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 U.S.-led coalition has in recent weeks escalated its aerial bombing campaign in northern Syria and Raqqa province. U.S.-backed forces have encircled the city of Raqqa and captured several districts from the militants.

This policy of engaging Syrian forces attacking our rebel allies should have been in place from day one.  A major reason that recruitment and training of these forces have gone so slowly is that the U.S. refused to protect the fighters we were arming.  President Obama's policy of refusing to engage Syrian forces even when they attacked U.S.-backed militias fighting ISIS played right into President Assad's hands.  He has been telling the world that his forces are "fighting terrorists," lumping anyone not loyal to the regime in with ISIS.  The Russians are pretending to agree with Assad and have done their share of damage to U.S.-backed forces.

But what happens next time if the planes are Russian?

President Putin knows that attacking our allies engaged with ISIS in the fight for Raqqa is a red line not to be crossed.  But Putin may have his own reasons to ensure that U.S.-backed fighters don't end up taking Raqqa.  The prestige of taking the ISIS capital would empower rebel forces, possibly prolonging the civil war. 

Raqqa is being attacked from three sides, with Hezb'allah militiamen and Syrian army units controlling several nearby towns and U.S. allies closing in from the north, west, and east.  The battle is expected to be long and bloody, with a rising toll of civilian casualties, as ISIS is preventing people from leaving.  The Pentagon says "hundreds" of U.S. soldiers are participating in the fight to liberate the city, but it is believed that most of the U.S. military efforts are directed toward intelligence, logistics, and long-range artillery. 

There is a possibility that as the pockets of resistance in Raqqa diminish, there will be confrontations between pro-Assad forces and the Arab-Kurdish coalition allied with the U.S.  It would be just one more open wound in Syria's civil war that has already killed at least 400,000.