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.

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.