The development came as the UN Security Council said it was waiting for Iran to honour an April 28 deadline for it to halt uranium enrichment and cooperate fully with inspectors from the UN’s International Atomic Energy Agency (IAEA).
The IAEA's director of safeguards Ollie Heinonen has decided not to travel to Iran following a visit to Tehran last week by IAEA chief Mohamed ElBaradei to seek a breakthrough in the Iranian nuclear crisis.
Iranians negotiating
An Iranian delegation was in Vienna this week negotiating with the IAEA on the agency's demands for Iran to suspend weapons related uranium enrichment and to cooperate fully with IAEA investigators.
Enrichment is a sensitive process as it can make either nuclear reactor fuel or atom bomb material. But Iran claims its program is a peaceful effort to generate electricity.
A diplomat said that whether Mr Heinonen went to Iran had "depended on whether the Iranians currently in Vienna give the IAEA anything new."
The IAEA seeks documents on dealings Iran had with a nuclear black market network run by disgraced Pakistani Abdul Qadeer Kahn, the father of his country's atomic bomb.
The agency also wants to interview military officers who may have overseen secret work that could be nuclear-weapons related.
The IAEA also wants more information on work Iran may have done on sophisticated P2 centrifuges, which can enrich uranium more quickly, as well as documents it has on making uranium hemispheres that form the core of atom bombs.
The IAEA currently has an inspection team at the Iranian enrichment facility in Natanz but the stakes are high as Mr ElBaradei is to submit a report next week to the UN Security Council on Iranian compliance.
Washington is pushing for moves that could lead to economic and other sanctions if Iran fails to comply but key Iranian allies and trading partners Russia and China are resisting such measures.
Cut off assistance
The United States says it will call for the UN atomic agency to cut off technical assistance to Iran if diplomatic efforts falter in getting Tehran to suspend uranium enrichment.
If Iran fails to meet the deadline, the United States wants the UN Security Council to adopt a "Chapter 7" resolution which would legally oblige Iran to meet the IAEA's calls for it to suspend enrichment and cooperate fully with an agency inspection of its nuclear program.
A Western diplomat said the US would use the June meeting of the IAEA's 35-nation board of governors to "cut off Iran's technical cooperation and call for an IAEA special investigation of the key unresolved safeguards concerns" about Iran's program if the Council fails to adopt a tough resolution.
The IAEA's technical cooperation with Iran is only about US$1.5 million a year, an IAEA official said.
Cutting off this aid "would certainly not punish Iran but it would complicate their dealings with the IAEA and be a public relations setback", one diplomat said.
The United States and Russia were at odds over how to proceed on the Iran issue, with Washington saying there was support for sanctions but Moscow denying this and ruling out the use of military force.