The United States said on Saturday Iran was "completely isolated" in its pursuit of an atom bomb, while talks at the UN nuclear agency stalled over what to demand of Tehran.
France, Britain and Germany formally submitted a toughly worded draft resolution to the UN International Atomic Energy Agency (IAEA) on Friday which called on Tehran to immediately freeze its uranium enrichment programme.
This is the most controversial part of Iran's programme since enrichment can produce nuclear material for weapons.
"The United States fully endorses the draft resolution. Iran remains completely isolated in its pursuit of nuclear weapons and the draft resolution to be considered this morning makes that clear," US Under Secretary of State John Bolton said.
The statement was read to reporters by the head of the US delegation at this week's meeting of the UN agency in Vienna, Jackie Sanders.
Usually IAEA resolutions are passed by consensus without a vote. But disagreement over the wording of the demand for Iran to suspend enrichment has led some countries to push for a vote.
"I think the draft of the EU three ... would have a majority" but no consensus, Iran's chief delegate to the IAEA meeting, Hossein Mousavian, told Iranian state television.
Several diplomats told Reuters that around five of the 13 non-aligned states on the board would vote against the draft. Negotiators from the EU trio were meeting with non-aligned diplomats to persuade them to accept the draft text.
Iran denies any plan to develop nuclear arms and insists its programme is intended only to produce electricity. It says its enrichment facilities would be used only to make low-enriched fuel for power plants, not highly-enriched fuel for bombs.
The draft's demand for the enrichment freeze has caused a split among the 35 members of the IAEA board of governors, some of which believe it could set a bad precedent.
Brazil and South Africa also have enrichment programmes and fear that one day they too could be told to freeze their commercial enrichment activities, diplomats said. They have the support of the large block of non-aligned states on the board.
Although Washington appears satisfied with the draft text, Western diplomats said US negotiators had to retreat somewhat from their demand that Iran be reported to the UN Security Council for hiding parts of its parts programme for 18 years.
"We've had very constructive negotiations," Bolton said about the week-long talks, which diplomats close to them described as occasionally heated.
A Western diplomat familiar with US thinking said he was confident the resolution, if approved, would lead to a Security Council referral and possibly economic sanctions in November.
Washington also failed to get an "trigger" clause in the text that would require a report to the Security Council if Iran failed to meet certain demands by an October 31 deadline.
"It looks like Iran's going to the Security Council," he said.
The IAEA has been investigating Iran's nuclear programme for two years. Although it has found many concealed activities that could be used to develop weapons, it has found no "smoking gun" that would prove US allegations of Iranian bomb plans.