Russia has moved jet fighters to a base in Syria for the first time, U.S. defense officials said Friday, a major military escalation that heightens fears Moscow is set to play a more direct role in propping up Syrian President Bashar al-Assad.

The deployment of a small number of tactical jets came just hours before U.S. Defense Secretary Ash Carter spoke to his Russian counterpart about Moscow’s deepening role in Syria, a call that ended a long pause in high-level military communication between the U.S. and Russia.

U.S. defense officials said the nearly hourlong conversation was designed in part to help avert a confrontation in Syria between the U.S. and Russia, which have long been on opposite sides of the 4½-year-old civil war.

Russia’s decision to send in jets, however, is the clearest indication Moscow is preparing to use military might to help Mr. Assad as he clings to power.

U.S. policy in Syria, which has for years sought to avoid getting drawn into the protracted and bloody conflict, is at a turning point, prompted by the flood of refugees into Europe and the surprising Russian escalation.