At risk is a possible American or Israeli military strike on Iran’s nuclear sites, or the Iranians following through on their threats to pursue an atomic weapon.

Mr. Trump’s chief envoy to the talks, Steve Witkoff, has also sent mixed signals over what the United States wants. Over the past week, he has moved from suggesting Iran could have a limited nuclear program to saying Iran needed to fully dismantle it.

A second round of nuclear talks between the United States and Iran is underway, as what both sides are looking for in a deal begins to take shape. Delegations from both countries met in Oman last weekend for talks mediated by the Gulf Arab nation.