The head of the UN nuclear agency has "serious concerns" that Iran may be hiding secret atomic weapons work and admits he failed in his latest attempt to unearth them.
International Atomic Energy Agency chief Yukiya Amano spoke to the 35-nation IAEA board amid backdoor diplomatic manoeuvring aimed at coming up with substantial joint pressure on Iran to end its nuclear defiance and address global concerns about its nuclear activities.
The conference opened as fears grow that Israel may soon strike Iran in an attempt to destroy its nuclear facilities. President Barack Obama is expected to ask Israeli Prime Minister Benjamin Netanyahu for restraint when they meet in Washington.
Mr Amano summarised the most pressing worries - Tehran's rebuff of two recent attempts to probe the weapons programme and a sharp, recent increase in uranium enrichment, which Iran says it needs for nuclear energy, but which can also produce weapons material.
The United States and its western allies are continuing to lobby Russia and China to back a resolution critical of Iran's refusal to heed IAEA and UN Security Council demands that it give full nuclear transparency.
Moscow and Beijing traditionally act as brakes on Western attempts to tighten sanctions on Iran.
Any resolution passed by the IAEA board automatically goes to the UN Security Council and could be used as a platform for additional sanctions on the Islamic Republic, which already is the focus of four sets of UN sanctions meant primarily to pressure it to give up enrichment. The US, the European Union and others have additionally slapped Tehran recently with financial and economic penalties meant to hurt its banking system and oil export industry.
Recent moves to boost higher-enriched enrichment at Fordo, an underground facility that may be able to withstand aerial attack, are of particular concern.
Referring to his most recent report on Iran circulated late last month, Mr Amano noted that Tehran had tripled higher monthly enrichment to 20% at Fordo over the past four months, as well as significantly expanding lower-level enrichment at another facility.
Both lower enriched uranium below 5% and 20% enriched material can be processed further to 90% - the level used to arm nuclear warheads. But 20% enrichment is of particular concern because it can be turned into weapons-grade material much more quickly and easily.
The IAEA meeting comes less than two weeks after IAEA experts returned from Tehran from their second failed attempt within a month to persuade Iran to end nearly four years of stonewalling on what the agency says is growing intelligence-based information that Iran has worked - and may still be working - on components of a nuclear weapons programme.