The diplomatic options for a solution to the Iranian nuclear crisis narrowed on Wednesday after a team of UN nuclear inspectors returned from Tehran without agreement on visiting a suspect site.
The International Atomic Energy Agency (IAEA) is due to issue its latest report on the Iranian nuclear programme on Friday, but took the unusual step of criticising Tehran's approach in a statement issued while the inspectors were still flying back to its headquarters in Vienna.
The main stumbling block was Iran's refusal to allow the IAEA team to visit a military site at Parchin, where the last agency report, issued in November, said there was a steel chamber which could have been used for testing explosives of a type performed in the development of a nuclear warhead.
"It is disappointing that Iran did not accept our request to visit Parchin during the first or second meetings," said the agency's director general, Yukiya Amano. "We engaged in a constructive spirit, but no agreement was reached."
Herman Nackaerts, the IAEA deputy director general and head of the safeguards department, who headed the mission, had made a Parchin visit the main litmus test for its success, according to diplomatic sources, but was rebuffed by the Iranians.
Speaking at Vienna airport on his return, Nackaerts said his team "could not find a way forward".
A Vienna-based diplomat briefed on the visit said Iran had sought to focus the talks on a work-plan circumscribing the conduct of IAEA inspections.
"It was very hard work. The Iranians focused exclusively on process and they tried to get the team to sign a document which governed the ways they would work," the diplomat said. "My reading is, what happened was that the meetings were monopolised by a lot of unproductive discussions on the wording of the agreement and practical questions put forward by the agency were put to the side."
The IAEA said: "Intensive efforts were made to reach agreement on a document facilitating the clarification of unresolved issues in connection with Iran's nuclear programme, particularly those relating to possible military dimensions. Unfortunately, agreement was not reached on this document."
In the wake of the collapse of the mission, Friday's report will almost certainly give a negative assessment of Iranian co-operation while noting the progress of the country's nuclear programme and uranium enrichment, which the UN security council has demanded Tehran suspend.
Iran insists it has a right to enrich uranium and the country's supreme leader, Ali Khamenei, put on a show of defiance on Wednesday with a rare meeting with Iranian nuclear scientists, insisting their work was peaceful, that Iran had no intention of building a bomb and vowing the programme would continue in the face of mounting international pressure.
"With God's help, and without paying attention to propaganda, Iran's nuclear course should continue firmly and seriously," Khamenei said on Iranian state television. "Pressures, sanctions and assassinations will bear no fruit. No obstacles can stop Iran's nuclear work."
Doubts have now been cast over tentative plans to hold a new round of talks between Iran and a six-nation group of major powers, including the five permanent members of the UN security council together with Germany. The group, known as the P5+1, had been waiting for the new IAEA report before deciding whether to proceed with the talks.
It was also seeking clarification on whether Iran had dropped its earlier preconditions for negotiations, which included an immediate end to sanctions and a guarantee that uranium enrichment was a non-negotiable Iranian right.
There had been hopes that the P5+1 meeting could agree confidence-building measures, possibly including an exchange of Iranian low enriched uranium for French-made fuel rods. Diplomats said the group would now have to reassess if there would be any purpose in a meeting.
Some western capitals are pushing instead for Iran to be referred to the UN security council by the IAEA board of member states, with the aim of imposing further sanctions. An EU oil embargo is already planned for 1 July, at about the same time of US financial sanctions against the Iranian global oil trade.