National Security Adviser Jake Sullivan speaks during a press briefing on Feb. 4 in the Brady Briefing Room of the White House in Washington, D.C.

The Biden administration is in contact with Iran to demand the release of Americans held in the Islamic republic, an issue that will help determine future relations between the countries, U.S. National Security Adviser Jake Sullivan said.

While Iran says it won’t negotiate unless the U.S. rejoins a nuclear accord abandoned by the Trump administration, Sullivan said “the script has been flipped” because President Joe Biden has offered to re-engage with Tehran.

“He’s prepared to go to the table to talk to the Iranians about how we get strict constraints back on their nuclear program,” Sullivan said on CBS’s “Face the Nation” on Sunday. “That offer still stands.”

“It is Iran that is isolated diplomatically now, not the United States, and the ball is in their court,” he said.

Asked about at least five U.S. citizens held by Iran, Sullivan said obtaining their release will be a “significant priority.”

“We have begun to communicate with the Iranians on this issue, and we will continue to do so as we go forward,” he said. “Our strong message to the Iranians will be that we will not accept a long-term proposition where they continue to hold Americans in an unjust and unlawful manner.”

Sullivan renewed U.S. calls for Chinese authorities and the World Health Organization to provide a fuller accounting of how COVID-19 virus originated and spread from China. He said U.S. officials “have questions about” a WHO report on the origins of the pandemic that followed months of negotiation with China to allow a fact-finding mission into the country.

“We do not believe that China has made available sufficient original data into how this pandemic began to spread both in China and then eventually around the world,” Sullivan said. “We believe both China and WHO should step up on this matter.

Trump has been out of the public eye for the past month, a stretch of time that included his acquittal by the U.S. Senate of an impeachment charge.

He emerged for phone-in interviews on conservative networks after last week’s death of broadcaster Rush Limbaugh from lung cancer. During those segments Trump, without prompting, repeated claims that the 2020 election had been “stolen” from him.

CPAC will highlight a split in the Republican Party between those loyal to Trump and others who want to move on — and who mostly won’t be at the meeting, which starts Thursday.

Asa Hutchinson, the Republican governor of Arkansas, said Sunday that Trump “will only define our party if we let him define our party” and that he wouldn’t support a Trump White House run in 2024.

“I think it’s fine for CPAC to invite former President Trump to speak, but how about the other voices — Senator (Bill) Cassidy from Louisiana, those that have different points of view, still arch-conservatives, but a different voice for the future of our party?” he said on CNN.

Former Texas GOP Rep. Will Hurd, who left Congress this year, said on NBC that Trump should have “very little if none at all” influence on the party going forward.

“This is a president that lost the House, the Senate, the White House in four years. I think the last person to do that was Herbert Hoover,” he said.

Bloomberg News