(CNN) -- U.S. Secretary of State John Kerry and Iranian Foreign Minister Mohammad Javad Zarif stepped to the sidelines for a private conversation at a security conference in Germany on Sunday, according to a senior State Department official.
Their discussion marks another high-level contact between the United States and Iran, a few weeks ahead of nuclear negotiations involving world powers planned for February 18 in Vienna. The negotiations follow Iran's agreement in November to stop its most sensitive nuclear operations in exchange for a lessening of some sanctions that have hurt the nation.
"Kerry reiterated the importance of both sides negotiating in good faith and Iran abiding by its commitments under the Joint Plan of Action," the nuclear deal between the U.S. and Iran, the senior official said.
Kerry "also made clear that the United States will continue to enforce existing sanctions," the official said.
On another topic, "Secretary Kerry pressed for the Iranians to work cooperatively with us in our efforts to help United States citizens Robert Levinson, Amir Hekmati, and Saeed Abedini to return to their families," the official said.
Levinson disappeared from Iran seven years ago. Members of Levinson's family said on CNN in January that, at that time, he was working for the CIA. They accused the U.S. government of failing to do enough to find and free him.
Hekmati, a former U.S. Marine who served in Iraq, was arrested in Iran in August 2011 and held on espionage charges. The Obama administration has said he is not a spy.
Saeed Abedini, an Iranian-American pastor, is also jailed, accused of proselytizing Christianity during a trip to Iran. He was accused of attempting to undermine the Iranian government and endangering national security. Abedini's family says he being held in a dangerous prison.
Hekmati and Abedini could receive reduced sentences, Zarif said in an interview less than two weeks ago from the World Economic Forum in Davos, Switzerland.
"We have various clemency measures in Iran that can be introduced, happened in the past, can be introduced again in these cases," Zarif said in an exclusive interview with CNN's Jim Sciutto.
Earlier discussions: Syria
In recent days at the Munich Security Conference, other contact between Kerry and Zarif has been focused on the conflict in Syria, which has killed more than 100,000 Syrians and forced more than 2 million from their homes.
Russian Foreign Minister Sergey Lavrov and U.N. Secretary-General Ban Ki-moon were also in Munich, as was U.N. mediator Lakhdar Brahimi. As talks continued, Syrian President Bashar al-Assad's regime failed to turn over its known chemical weapons stockpiles on time, causing Kerry to warn on Friday that all options remain available to force compliance. Iran, Russia and Syria are allies.
U.S. President Barack Obama threatened a military response last year when U.S. authorities determined the al-Assad regime attacked its own people with chemical weapons. However, Obama failed to secure backing from Congress or key ally Great Britain.
Russia then stepped in to persuade al-Assad to hand over the chemical weapons stockpiles to the international community in order to be destroyed.
On Saturday, Kerry and Turkish Foreign Minister Ahmet Davutoglu met on the sidelines of the security conference and talked about continuing to press the Syrian regime to meet the deadline and move chemical weapons to the port of Latakia in Syria, a senior State Department official said.
"They agreed that more needed to be done to increase humanitarian access," the official said, adding that the Kerry and Davutoglu said that the delay of moving access to convoys into the besieged Syrian city of Homs and other areas was "simply unacceptable."
CNN's Elise Labott contributed to this report.