WASHINGTON — Russian aircraft carried out a bombing attack against Syrian opposition fighters on Wednesday, including at least one group trained by the C.I.A., eliciting angry protests from American officials and plunging the complex sectarian war there into dangerous new territory.

Russia’s entry into the Syrian conflict, foreshadowed by a rapid military buildup in the past three weeks at an air base in Latakia, Syria, makes the possibility of a political settlement in Syria more difficult and creates a new risk of inadvertent incidents between American and Russian warplanes flying in the same area. And it adds a powerful but unpredictable combatant to a civil war that has already resulted in hundreds of thousands of deaths and a flood of refugees.

President Vladimir V. Putin of Russia justified his country’s entry into the conflict by saying that Russia was acting “preventatively, to fight and destroy militants and terrorists on the territories that they already occupied, not wait for them to come to our house.”

But American officials said the attack was not directed at the Islamic State but at other opposition groups fighting against the government of the Syrian president, Bashar al-Assad, whom Mr. Putin has vowed to support. American officials said Russian warplanes and helicopter gunships had dropped bombs north of the central city of Homs, where there are few, if any, militants of the Islamic State, also known as ISIS or ISIL.