CAIRO — In the clearest statement yet of the United States’ position on the military’s ouster of President Mohamed Morsi, a senior American diplomat warned on Monday that the generals would jeopardize Egypt’s “second chance” at a democratic transition if their new interim government continued to crack down on Mr. Morsi’s Islamist supporters.

“If representatives of some of the largest parties in Egypt are detained or excluded, how are dialogue and participation possible?” the diplomat, Deputy Secretary of State William J. Burns, told journalists after meeting with generals and the interim officials they have appointed.

“It is hard to picture how Egypt will be able to emerge from this crisis unless its people come together to find a nonviolent and inclusive path forward,” Mr. Burns said.

Mr. Burns, the first senior United States official to visit Cairo since the takeover, spoke against the backdrop of a standoff between the military’s interim government and tens of thousands of Islamists who have staged a two-week sit-in to protest the ouster of Mr. Morsi, Egypt’s first freely elected president.