Russia launched airstrikes in Syria on Wednesday, catching U.S. and Western officials off guard and drawing new condemnation as evidence suggested Moscow wasn’t targeting extremist group Islamic State, but rather other opponents of Bashar al-Assad’s regime.

One of the airstrikes hit an area primarily held by rebels backed by the Central Intelligence Agency and allied spy services, U.S. officials said, catapulting the Syrian crisis to a new level of danger and uncertainty. Moscow’s entry means the world’s most powerful militaries—including the U.S., Britain and France—now are flying uncoordinated combat missions, heightening the risk of conflict in the skies over Syria.

U.S. Defense Secretary Ash Carter said Russia’s approach to the Syrian war—defending Mr. Assad while ostensibly targeting extremists—was tantamount to “pouring gasoline on the fire.”…

The U.S. and its allies were angry at the Russians on many scores: that they are supporting Mr. Assad; that they aren’t coordinating their actions with the existing, U.S.-led anti-Islamic State coalition; that they provided terse notice only an hour before their operations; that they demanded the U.S. coalition stay out of Syrian airspace; and that they struck in areas where anti-Assad rebels—not Islamic State—operate.