Secretary of State John F. Kerry told Iran’s foreign minister Sunday that the United States will continue to enforce existing sanctions on Iran while bargaining over a deal to rein in Iran’s disputed nuclear program.

The top U.S. and Iranian diplomats held a rare face-to-face meeting Sunday in Germany, the State Department said. The private meeting furthers a tentative warming between the two nations that began with the election last year of Iranian President Hassan Rouhani.

Kerry’s discussion with Iranian Foreign Minister Mohammad Javad Zarif was their first since the United States and Iran struck a temporary agreement that caps the most worrisome elements of Iran’s nuclear program in exchange for limited easing of global financial restrictions on Iran’s oil business. They also met in September at the United Nations to begin talks that Iran has sought as a way to end crushing economic sanctions.

“Secretary Kerry reiterated the importance of both sides negotiating in good faith and Iran abiding by its commitments” under that initial agreement, a senior State Department official said Sunday. “He also made clear that the United States will continue to enforce existing sanctions.”

The official spoke on the condition of anonymity to describe the closed-door meeting on the sidelines of an international security conference. The meeting lasted slightly longer than an hour, the official said.

The nuclear negotiations with Iran are politically sensitive at home, where many in Congress oppose the Obama administration’s strategy of limited easing of sanctions imposed in protest of a secretive nuclear program that Iran says has no military purpose.

Israeli Prime Minister Benjamin Netanyahu also opposed the interim, six-month deal as a giveaway to Iran. The United States and Israel have accused Iran of trying to build a nuclear weapon.

European Union foreign policy chief Catherine Ashton, who is leading negotiations with Iran on behalf of six world powers, called the one-on-one meeting “incredibly important” to build confidence for the larger negotiations set to begin Feb. 18 in Vienna.

The goal is a comprehensive agreement that ends a 10-year diplomatic impasse and ensures that Iran cannot quickly redirect its advanced nuclear development work to build a bomb. Kerry has repeatedly said negotiations will be extremely difficult.

Kerry and Zarif have portrayed the interim deal reached in November very differently for their respective publics. Kerry stresses that the deal forces Iran to stop uranium enrichment work considered the most likely to lead to a bomb and degrade its existing stocks of the most potent uranium. Zarif stresses the economic benefit to Iran and what he calls a recognition of Iran’s right to a continued homegrown uranium enrichment program.

Zarif said in an interview Saturday that Iran was not prepared to give up research on centrifuges used to purify uranium as part of a final nuclear deal.

“We are ready because we believe it is in our interests and we have no other intention. So theoretically it shouldn’t be that difficult,” Zarif told Reuters and International Media Associates, a television production company.

In Iran, Mehdi Karroubi, the 76-year-old former speaker of Iran’s parliament, was released from custody. The move is the first sign that Karroubi and former prime minister Mir Hossein Mousavi may be exonerated for leading the 2009 “Green Movement.”

Both men were accused of inciting massive street protests after the disputed reelection of President Mahmoud Ahmadinejad in 2009. Since the election of Rouhani as Iran’s president in June, there have been growing calls from reformists for the release of Mousavi and Karroubi.

Karroubi has been held under arrest in a heavily guarded state-controlled residence since 2011.

But in an interview Sunday with the official Iranian Student News Agency, Karroubi’s son, Hossein Karroubi, said that though his father returned home Saturday night, the elder Karroubi’s fate is still unclear.

“The groundwork for this transfer was done 10 days ago, and now he is living on the second floor and the security team are in a separate suite on the first floor. The security situation has not changed, just the location,” Hossein Karroubi said.

Jason Rezaian in Tehran contributed to this report.