Beefing up international monitoring of Iran’s nuclear work could become the biggest stumbling block to a final accord between Tehran and major powers, despite a preliminary deal reached last week.

As part of that deal, Iran and the powers agreed that UN inspectors would have “enhanced” access to remaining nuclear activity in Iran, where they already monitor key sites.

But details on exactly what kind of access the inspectors will have were left for the final stage of talks, posing a major challenge for negotiators on a complex and logistically challenging issue that is highly delicate for Iran’s leaders.

Securing proper inspections is crucial for the US and other Western powers to ensure a final deal, due by June 30, is effective and to persuade a sceptical US Congress and Israel to accept the agreement. Iran says its nuclear programme is peaceful, but it has never welcomed intrusive inspections and has in the past kept some nuclear sites secret.

Securing proper inspections is crucial for the US and other Western powers to ensure a final deal

Sharply differing interpretations have emerged on what was covered by last week’s framework agreement – a sign of what diplomats and nuclear experts say will be tough talks ahead.

Iranian Supreme Leader Ayatollah Ali Khamenei, who has the final say for Iran on the deal, on Thursday ruled out any “extraordinary supervision measures” over nuclear activities and said military sites could not be inspected.

That appeared to contradict a US “fact sheet” issued after last week’s marathon talks in Switzerland which said the UN International Atomic Energy Agency (IAEA) would have “regular access to all Iran’s nuclear activities” and to the supply chain that supports it, as well as a joint Iranian-European Union statement that said the IAEA would have expanded access in Iran.

Aside from the question of Iranian consent, the logistical requirements for increased monitoring of Iranian sites would be daunting. It would involve more cameras, on-site inspections, satellite surveillance and other methods and might require the IAEA to assign more people and resources to its Iran team.

David Albright, president of the Institute for Science and International Security, said it was crucial to come up with a mechanism for “anytime, anywhere” inspections that go beyond the IAEA’s own special arrangements for short-notice inspections.