WASHINGTON — President Obama’s administration threatened Wednesday to suspend plans for coordinating counterterrorism strikes in Syria with Russia unless Moscow moves to stop its assault on the city of Aleppo and restore a collapsed cease-fire.

Secretary of State John Kerry, in a telephone call to his Russian counterpart, “made clear the United States and its partners hold Russia responsible for this situation,” including the “drastic escalation” of its military actions in recent days and refusal to ensure the safe passage of humanitarian aid to government-besieged areas, the State Department said.

The United States, Kerry told Foreign Minister Sergei Lavrov, “is making preparations to suspend U.S.-Russia bilateral engagement on Syria …. unless Russia takes immediate steps to end” the heightened carnage in Aleppo, where at least a quarter-million civilians are trapped amid withering Syrian and Russian bombing.

The scale of the slaughter is unprecedented in Syria’s five years of civil war, as are the international cries for something to be done. Yet there is a growing sense that the United States is powerless to stop it, or is at least unwilling to take steps that might force Moscow and Damascus to change their calculations.