U.S. Secretary of State John Kerry and Iranian Foreign Minister Mohammed Javad Zarif met in Geneva on Saturday for almost six hours, and talks are set to resume in Vienna among technical experts within a few days.

The hoped-for accord would curb Iran’s nuclear program in exchange for an easing of economic sanctions. Zarif and Kerry are expected to meet again within a couple of weeks, before entering into the most critical phase in the days before the deadline.

Saturday’s discussions between the top diplomats were focused and, at times, intense, according to officials, who spoke under rules requiring anonymity. Zarif and Kerry reflected a sense of urgency to make key decisions, the officials said. U.S. Energy Secretary Ernest Moniz and the EU’s Stephan Klement, both nuclear experts, took part in the talks as did EU negotiator Helga Schmid.

Kerry and Zarif, along with their teams, had a thorough and comprehensive discussion of all of the issues, said a senior State Department official.

Very Detailed

The meeting followed several weeks of technical-level talks among aides. It was the first Kerry-Zarif negotiating session since Iran and six world powers agreed in early April to a framework that set the deadline to conclude a full accord.

Kerry has mostly cleared his travel schedule for June to be available when needed for the talks on curbing Iran’s nuclear program and easing sanctions against the Islamic Republic.

The talks on Saturday were very detailed and Iran’s top nuclear specialist, Ali Akbar Salehi, the head of the Iranian Atomic Energy Organization, participated by phone because he is ill in Tehran, according to U.S. officials.

One of the fundamental remaining differences is that Iran wants a quick easing of sanctions after an agreement, while the U.S. insists most sanctions relief take effect only after Iran takes actions that are verified.

Deal-Killing Stance

There are also questions of transparency and verification, particularly concerning access to Iran’s military facilities and interviews with its nuclear scientists.

Iran has told its negotiating partners that Supreme Leader Ayatollah Ali Khamenei has drawn a red line ruling out both, Deputy Foreign Minister Abbas Araghchi said on Iranian television. U.S. officials say that stance would be a deal- killer if it were it to hold.

Iranian officials have said the June 30 deadline could slip. Without closing the door to that possibility, a State Department official said the deadline is important as a mechanism forcing action on the hard decisions that remain.

A deal intended to deny Iran the capability to build a nuclear weapon for at least a decade, and possibly two, would be a major legacy success for Kerry and his boss, President Barack Obama.

Congressional Skeptics

Any any accord will face review by Congress, where many members are skeptical that Iran would honor a deal. They’re also mindful of opposition from Israeli Prime Minister Benjamin Netanyahu, who has said it’s already clear that the terms would be too lenient on Iran.

Issues of access in Iran concern both future actions and longstanding questions about what the International Atomic Energy Agency calls the possible military dimension of past activities by Iran. It includes the Parchin military base where the nuclear watchdog has been denied access.

The negotiations involve Iran and the U.S., U.K., France, Germany, Russia and China. A final deal would require approval from all of the parties.