The UN nuclear watchdog is expected this week to issue its most detailed report yet on research in Iran seen as geared to developing atomic bombs, heightening international suspicions of Tehran's agenda and stoking Middle East tensions.
Western powers are likely to seize on the International Atomic Energy Agency document, which has been preceded by media speculation in Israel of military strikes against Iranian nuclear sites, to press for more sanctions on the oil producer.
But Russia and China fear the publication now of the IAEA's findings could hurt any chance of diplomacy resolving the long-running nuclear row and they have lobbied against it, signalling opposition to any new punitive UN measures against Iran.
Iran rejects allegations of atomic weapons ambitions, saying its nuclear programme is aimed at producing electricity. The report is tentatively scheduled to be submitted to IAEA member states on November 9 before a quarterly meeting the following week of the agency's 35-nation board of governors in Vienna.
It "will be followed by a US-European Union push for harsher sanctions against Iran at the UN Security Council, where Western powers will meet stiff resistance from Russia and China," said Trita Parsi, an expert on US-Iran relations.
The document is expected to give fresh evidence of research and other activities with little other application than atomic bomb-making, including studies linked to the development of an atom bomb trigger and computer modelling of a nuclear weapon. Sources briefed on the report also say it will include information from both before and after 2003 - the year in which US spy services estimated, in a controversial 2007 assessment, that Iran had halted outright "weaponisation" work.
Many conservative experts criticised the 2007 findings as inaccurate and naive, and US intelligence agencies now believe Iranian leaders have resumed closed-door debates over the last four years about whether to build a nuclear bomb. "The primary new information is likely to be any work that Iran has engaged in after 2003. Iran is understood to have continued or restarted some research and development since then," said Peter Crail of the Arms Control Association, a US-based advocacy group.
The sources familiar with the document said that among other things it would support allegations that Iran built a large steel container for the purpose of carrying out tests with high explosives applicable to nuclear weapons.
"This is not a country that is sitting down just doing some theoretical stuff on a computer," a Western official said about the IAEA's body of evidence, which is based on Western intelligence as well as the agency's own investigations.
The report will flesh out and expand on concerns voiced by the IAEA for several years over allegations that Iran had a linked programme of projects to process uranium, test high explosives and modify a missile cone to take a nuclear payload.