Washington — White House national security adviser Jake Sullivan said Sunday that the U.S. has begun to communicate with Tehran about Americans who are being held hostage in Iran and said President Biden is “prepared to go to the table” to speak with the Iranians about constraints on their nuclear program.

In an interview with “Face the Nation,” Sullivan called the detention of Americans in Iran a “complete and utter outrage” and “humanitarian catastrophe,” and said the release of those hostages will be a top priority for the Biden administration.

“We have begun to communicate with the Iranians on this issue, yes, and we will continue to do so as we go forward,” Sullivan said. “And 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.”

• Transcript: National Security Adviser Jake Sullivan on “Face the Nation”

Secretary of State Antony Blinken has spoken with the families of Americans being held hostage or are wrongfully detained abroad, and at least four Americans with dual citizenship are imprisoned in Iran.

On Thursday, the Biden administration took a significant step toward restoring the 2015 Iran nuclear deal, which former President Donald Trump withdrew the U.S. from in 2018. Unlike his predecessor, Mr. Biden vowed during the presidential campaign to rejoin the agreement, which aims to limit Iran’s nuclear program.

Sullivan said the president is “determined” to stop Iran from getting a nuclear weapon.

“He believes that hardheaded, clear-eyed diplomacy is the best way to do that,” Sullivan said of Mr. Biden. “And so he’s prepared to go to the table to talk to the Iranians about how we get strict constraints back on their nuclear program. That offer still stands because we believe diplomacy is the best way to do it. Iran has not yet responded. But what’s happened as a result is that the script has been flipped. It is Iran that is isolated now diplomatically, not the United States. And the ball is in their court.”

Sullivan also discussed the sweeping SolarWinds hack, in which network management software used by the U.S. government and the private sector was breached. Roughly 100 private-sector companies and nine federal agencies were compromised, deputy national security adviser for cyber and emerging technology Anne Neuberger said Wednesday. Neuberger said Russia is likely behind the cyber intrusion.

Sullivan said the White House has asked the intelligence community to do more work to sharpen the attribution made by the Trump administration, including about how the hack occurred, the extent of the damage, and the scope and scale of the breach.

“It will be weeks, not months, before we have a response prepared,” he said. “That response will include a mix of tools seen and unseen. And it will not simply be sanctions because, as you say, a response to a set of activities like this requires a more comprehensive set of tools, and that is what the administration intends to do.”

Sullivan said the Biden administration is in the process of working through how best to respond to the intrusion.

“We will ensure that Russia understands where the United States draws the line on this kind of activity,” he said.

Mr. Biden raised the SolarWinds hack with Russian president Vladimir Putin when the two spoke last month. Neuberger said the president is expected to take executive action to address cybersecurity gaps identified in a review of the breach.