A vote by the International Atomic Energy Agency's 35-nation board to report Iran to the Security Council has been deferred until tomorrow.
The board is now due to reconvene at 09:00 Irish time tomorrow.
The move looks increasingly likely after Russia and China conditionally endorsed an EU-sponsored resolution to put the council on notice about Iran.
Both countries' support is contingent on Tehran being given at least until March to co-operate fully with UN investigators before any action can be taken.
Iran has threatened to respond by curbing UN inspections and ending talks on Russia's offer to enrich Iranian uranium to ensure the Islamic Republic cannot divert it for bombs. Tehran says it only wants nuclear power for electricity.
The International Atomic Energy Agency's board of governors began a crisis session yesterday to debate the EU motion but adjourned for informal talks that EU and US leaders hoped would yield a maximum tally of "yes" votes.
Analysts expected a majority of 25-30 in favour of the resolution, with opposition only from adversaries of Washington such as Cuba and Venezuela.
Western diplomats said a rare consensus among permanent council powers Britain, China, France, Russia and the United States, plus Germany, to report Iran was helping to win over wary developing states on the IAEA board.
Tehran told IAEA chief Mohamed ElBaradei that any Security Council involvement "would be the final blow to the confidence of Iran" in the UN watchdog, which would be barred from carrying out spot checks on Iranian nuclear facilities.
Losing the ability to make short-notice inspections would impair the IAEA safeguards regime in Iran, but EU diplomats said the threat would not deter efforts to rein in Tehran.