Echoing several other top U.S. officials in recent days, John F. Kirby, a Defense Department spokesman, noted to reporters on Friday that Afghan government forces had numerical and firepower advantages over the Taliban and suggested they should be able to stop the onslaught if only they tried hard enough.

“We will do what we can from the air, but they have the advantage. They have greater numbers. They have an air force. They have modern weaponry,” he said. “It’s time now to use those advantages.”

A day earlier, Mr. Kirby was more explicit. “No potential outcome has to be inevitable, including the fall of Kabul,” he said. “It doesn’t have to be that way. It really depends on the kind of political and military leadership that the Afghans can muster to turn this around.”

President Biden has spoken in similar terms, saying on Tuesday that Afghan forces “have got to want to fight.”