The hours are ticking down to what the Syrian government and its main ally, Russia, say could be the most devastating aerial assault yet on besieged rebel-held districts in the Syrian city of Aleppo.

The Obama administration has offered no military lifeline even to rebel groups it has vetted and backed.

But Donald J. Trump, the American president-elect, has gone a step further, at least in his remarks, suggesting that he will end all support to rebels and perhaps even treat the Syrian and Russian governments as allies in the fight against the Islamic State.

Some rebels and civilian supporters say such a move might not make much practical difference, and would at least put the American position out in the open, instead of hiding it behind condemnations of Bashar al-Assad, the Syrian president.