MOSCOW — Turkey’s confrontation with Syria spread on Thursday to include Russia, Syria’s principal military ally, when Turkey’s prime minister said Russian munitions intended for Syria’s government had been impounded from a Syrian commercial jetliner forced to land in Turkey.

Syria and Russia protested the interception and grounding of the jetliner. Turkish warplanes forced it to land on Wednesday on suspicion of transporting war matériel while en route from Moscow to Damascus with 35 passengers, including a number of Russians. Syria accused the Turks of assaulting the crew, denied that any illegal cargo had been aboard and demanded the return of whatever had been seized.

The developments aggravated the combustible atmosphere enveloping the conflict in Syria, where a 19-month-old uprising against President Bashar al-Assad has turned into a civil war that threatens to destabilize the Middle East. Turkey is a major backer of the insurgents trying to topple Mr. Assad and has hinted it may take military action against his forces because of the conflict, which has sent more than 100,000 Syrian refugees into Turkey. Russia is the major arms supplier to Mr. Assad’s government.

Fighting between Syrian insurgents and Mr. Assad’s forces convulsed northern Syria near the Turkish border, with unconfirmed reports that rebels had seized control of a strategic highway into the embattled city of Aleppo that the Syrian Army used to resupply its troops.