WASHINGTON — The lead US negotiator said today that transatlantic consultations on toughening policy toward Iran were making progress, but he offered no guarantees that US President Donald Trump would accept any agreement that might be reached.

“First we have to reach an agreement with the Europeans,” Brian Hook, the State Department policy planning chief, said on a call with journalists today. “If we can reach an agreement, then that will be presented to the president by the secretary of state and the national security adviser, and then he will make a decision on whether he wants to remain in the [Iran nuclear] deal or stop waiving sanctions.”

Trump vowed in January that he would not renew US nuclear-related Iran sanctions waivers at the next deadline of May 12 unless the Europeans agree to a list of demands. His demands include a more robust interpretation of the International Atomic Energy Agency mandate to inspect Iran sites; preventing Iran from developing long-range intercontinental ballistic missiles; and, most problematic for reaching a US-European understanding, extending the duration of strict limits on Iran’s enrichment capacity indefinitely. Under the terms of the Joint Comprehensive Plan of Action (JCPOA), those limits ease — or “sunset” — after a decade to allow Iran to expand its enrichment capacity.

“We have had constructive talks with the Europeans toward a supplemental agreement,” Hook said. “But I cannot predict whether we will reach an agreement with them or not.”