On Tuesday, the eve of the 36th anniversary of the student takeover of the American Embassy in Tehran, state television announced the arrest of a Lebanese-American missing for weeks — after he had been invited here by the government. He has been accused of spying.

Ayatollah Ali Khamenei, the supreme leader, said the "Death to America" slogan is eternal. New anti-American billboards in Tehran include a mockery of the Iwo Jima flag-raising photograph that symbolized Marine sacrifice in World War II. And an Iranian knockoff version of K.F.C., the chicken chain widely associated with the United States, was summarily closed after two days.

"It feels like a witch hunt," said one Iranian-American businessman in Tehran, who dared not speak for attribution over fear for his safety. "It's pretty scary."

Ever since the nuclear accord was reached in mid-July and endorsed byAyatollah Khamenei, he has been insisting it did not signal rapprochement with the United States — although some tacit improvements have emerged.

Military forces of Iran and the United States have avoided each other in fighting Islamic State militants in Iraq. Last week, Iran participated for the first time in international talks aimed at resolving the Syria conflict.