A month from a nuclear deal deadline, the top US and Iranian diplomats gathered in Geneva yesterday in an effort to bridge differences over how quickly to ease economic sanctions on Tehran and how significantly the Iranians must open up military facilities to international inspections.

The talks between US Secretary of State John Kerry and Iranian Foreign Minister Mohammad Javad Zarif were likely to extend into today, a negotiating round that officials described as the most substantive since world powers and Iran clinched a framework pact in April.

That agreement, however, left big questions unanswered, which weeks of subsequent technical discussions have done little to resolve.

Asked about completing the full accord by June 30, Zarif said yesterday: “We will try.”

The agreement left big questions unanswered

World powers believe they have secured Iran’s acquiescence to a combination of nuclear restrictions that would fulfil their biggest goal: keeping Iran at least a year away from bomb-making capability for at least a decade. But they are less clear about how they’ll ensure Iran fully adheres to any agreement.

Various Iranian officials, including Ayatollah Ali Khomenei, the supreme leader, have publicly vowed to limit access to or even block monitors from sensitive military sites and nuclear scientists suspected of previous involvement in covert nuclear weapons efforts.

The US says such access must be guaranteed or there will be no final deal. A report on Friday by the UN nuclear agency declared work essentially stalled on its multi-year probe of Iran’s past activities.