He did not say how long that waiver should last, and his appeal has made little progress.

Mr. Trump said that he would be willing to give some medical equipment to Iran to combat the virus, such as ventilators, “if they ask for it.” Iran’s leaders have not asked.

Trump administration officials say their threat to return to the far harsher sanctions — which blocked virtually all oil sales and drove Iran to the negotiating table — would not come until fall, presumably after the first phase of the coronavirus response has passed. They maintain it is separate from any relaxation of restrictions on medical supplies, some of which are exempted already from U.S. sanctions.

The arms embargo at the center of the dispute was something of a sideshow to the main nuclear agreement. The agreement covers only Iran’s nuclear activity: It required Iran to ship about 97 percent of its nuclear fuel out of the country — moved to Russia, in early 2016 — and to observe sharp limits on its production of nuclear material for 15 years.

Iran abided by those limits for a year after Mr. Trump pulled out of the agreement. But since last summer, it has gradually violated the limitations on both how much nuclear fuel it is allowed to stockpile and the level to which it can enrich its fuel. As a result, experts agree that it has greatly shortened its “breakout time,” the period needed to make enough fuel for a single nuclear weapon. Iran insists it would return to the agreed-upon levels as soon as Mr. Trump came back into compliance with the agreement by lifting unilateral sanctions.

The arms embargo — along with limits on missile launches — was part of a United Nations Security Council resolution that enshrined the nuclear accord, and suspended years of U.N.-imposed sanctions. That is what begins to expire in October. (The limits come off in stages: Small arms restrictions end this year, but restrictions on missiles and their components remain in place for another three years.)

Mr. Pompeo has warned that if the weapons embargo is not extended, Iran will supply more arms to groups that the United States considers terrorists, including Syrian militias and Hezbollah. Even his critics on Capitol Hill agree that he is probably right about what would happen to those arms.

Wendy R. Sherman, who served as the negotiation team leader of the Iran accord during the Obama administration and now directs the Center for Public Leadership at Harvard, recalled that the Russians and Chinese never wanted a conventional arms embargo on Iran, and only agreed to one of limited duration.