BEIRUT, Lebanon — At a summit meeting in Tehran on Friday, the presidents of Russia, Turkey and Iran agreed that “there could be no military solution” to the war in Syria, but Russia rejected Turkey’s call for a cease-fire while Iran called for a military push to crush the Syrian rebels and drive out American forces.

At the same time, at a meeting of the United Nations Security Council in New York, Western powers warned of unspecified consequences if Syria and its allies launched an offensive that has raised fears of a humanitarian disaster, but they cannot act without the assent of Russia, which backs the Syrian government.

Both meetings, while seemingly fruitless, starkly illustrated the extent to which Syria’s fate now lies in the hands of foreign powers and offered little hope that those powers could resolve the crisis diplomatically.

Speaking at the Security Council, Staffan de Mistura, the United Nations envoy for Syria, said the military offensive was incompatible with diplomatic efforts.