“With respect to Assad, there is a political reality that we have to accept,” said Sean Spicer, the White House press secretary.

“The United States has profound priorities in Syria and Iraq, and we’ve made it clear that counterterrorism, particularly the defeat of ISIS, is foremost among those priorities,” he added, using the acronym for the Islamic State.

In a sense, Mr. Spicer’s comments — and similar comments on Thursday by Secretary of State Rex W. Tillerson and Nikki R. Haley, the United States ambassador to the United Nations — have merely made explicit an assumption that has guided the Trump administration’s policy toward the region in recent months.

But the comments have also stirred criticism, including from some Republican lawmakers, who assert that Syria will continue to be a magnet for extremists as long as Mr. Assad is in control of the country.