"Whether the president is Person A or Person B is less relevant than whether their system is prepared to make commitments to restrain their nuclear program," Jake Sullivan said on ABC's "This Week."

His comments followed the election Friday in Iran of ultraconservative cleric Ebrahim Raisi as president, succeeding moderate Hassan Rouhani -- for whom the nuclear deal was a landmark achievement -- and they come as talks in Vienna on rescuing the nuclear deal stand at a potential turning point.

"The ultimate decision for whether or not to go back into the deal," Sullivan said, "lies with Iran's supreme leader."

The 81-year-old Ayatollah Ali Khamenei has long stood as the ultimate arbiter of Iran's strategic posture. Raisi is considered a close Khamenei loyalist.

Under former president Donald Trump, the United States quit the Iran nuclear deal in 2018 and imposed new sanctions on Tehran.

His successor Joe Biden has said he wants to return to the accord as a crucial way of reining in Iran's nuclear program.

The multination talks in Vienna, underway since April, aim to bring the US back into the deal and to persuade Tehran to again abide by curbs on its nuclear program while providing Iran with sanctions relief.

Israel, a close US ally and bitter foe of Iran, has been fiercely critical of the nuclear deal.

Israel's new premier Naftali Bennett Sunday described Raisi's victory as "the last chance for the world powers to wake up before returning to the nuclear agreement."

But Enrique Mora, top European Union diplomat at the Vienna talks, said Sunday that negotiators were "closer" to saving the deal, despite some sticking points.

Sullivan also expressed cautious optimism.

"There is still a fair distance to travel on key issues, including sanctions and commitments Iran has to make," he said.

But, Sullivan added, "The arrow has been pointed in the right direction... We'll see if Iranian leaders are prepared to make the hard choices."