U.S. Secretary of State John Kerry, left, and Iranian Foreign Minister Mohammad Javad Zarif at U.N. headquarters Thursday. (Jason DeCrow/AP)

The top diplomats from the United States and Iran met privately Thursday on the sidelines of a larger negotiating session about Iran’s disputed nuclear program, marking the highest-level discussion in years between two nations that have been firm adversaries for more than three decades.

Both U.S. Secretary of State John F. Kerry and Iranian Foreign Minister Mohammad Javad Zarif called their talks productive and constructive. The group of nations that have negotiated intermittently with Iran over its nuclear ambitions agreed to meet again in about three weeks in Switzerland for what Kerry called more-substantive work.

“I think all of us were pleased that Foreign Minister Zarif came and made a presentation to us, which was very different in tone and very different in the vision that he held out, with respect to the possibilities for the future,” Kerry told reporters.

Kerry did not detail what Zarif had proposed but said further sessions would seek to “find a way to answer the questions that people have about Iran’s nuclear program.”

Zarif, speaking in English, followed Kerry to the podium and said he welcomed the opportunity to try to address international doubts about Iran’s nuclear intentions.

“We hope to be able to make progress toward resolving this issue in a timely fashion based on respecting the rights of the Iranian people to nuclear technology for peaceful purposes,” Zarif said, “and at the same time making sure that there is no concern at the international level that Iran’s nuclear program is anything but peaceful.”

The United States, Israel and several other nations have strong suspicions that Iran is using its development of a domestic nuclear energy program as a cover to amass the means and know-how to build a nuclear weapon.

The U.N. Security Council — and separately the United States and European Union — has imposed strict economic sanctions on Iran for failing to comply with past demands for information and assurances, and the new Iranian leadership is making a case for easing those crippling measures.

“Sanctions are counter­productive,” Zarif said. “As we move forward there has to be removal of sanctions and in the end­game a total lifting of all sanctions.”

No U.S. secretary of state had met directly with an Iranian counterpart since 2007, and that session amounted only to pleasantries at a dinner.

Kerry and Zarif sat beside each other during the main meeting and met with only a handful of aides for several minutes afterward. They emerged separately to speak to reporters.

“Needless to say, one meeting and a change in tone, which was welcome, doesn’t answer those questions yet,” Kerry said. “We hope very, very much, all of us, that we can get the concrete results that will answer the outstanding questions.”