In advance of a meeting on Tuesday, diplomats said the US was not trying at the meeting of the International Atomic Energy Agency's (IAEA) board of governors, that began in Vienna on Monday, to get Iran referred to the UN Security Council for possible sanctions.
Washington still wants, however, to get the idea of applying the trigger mechanism for punishing Iran into its proposed resolution.
The US charges Iran of secretly developing nuclear weapons, but Tehran maintains its nuclear programme is strictly peaceful.
At the last IAEA board meeting in November, the US backed off from threatening sanctions, but won a guarantee - the so-called trigger mechanism - that the council be alerted if Iran's violations continue.
But, said one diplomat, "the March meeting is not the time to present a text to apply" this measure.
The diplomat said the IAEA should be trying to encourage Iran's cooperation with the nuclear watchdog, even while it criticised the Islamic republic for its continuing breaches of the nuclear non-proliferation treaty.
Another diplomat said: "This meeting in March does not carry great weight. It is an interim situation, an interim resolution."
Even US ambassador to the IAEA Kenneth Brill, who condemned Iran for "changing its stories to fit the facts" in order to hide a continuing nuclear weapons programme, echoed this when he told reporters on Monday: "This board has a lot more work to do and I expect will be dealing with the Iranian issue for many boards to come."
Al-Baradai says the IAEA will
continue probing Iran's nukes
The bottom line is that the United States does not have the backing at the IAEA board, which includes non-aligned states, to take the matter to the Security Council.
Britain, France and Germany in October had struck a deal with Iran to cooperate with the IAEA, and are still stressing the path of "constructive engagement", a Western diplomat said.
He said the United States has drafted a resolution against Iran for hiding sensitive parts of its nuclear programme as it wants to find a "way of keeping the pressure on" Tehran to fully disclose its activities.
Europeans are watching carefully the wording of the resolution, in order not to go too far in condemning the Iranians, he said.
"The US text is not balanced. Washington acts as if nothing positive has happened since November" despite Iran's allowing wider inspections by the IAEA, another diplomat said.
IAEA chief Muhammad al-Baradai has said on Monday the agency will continue probing charges that Iran is secretly developing nuclear weapons, despite Tehran's insistence the investigation should end as it has fully disclosed its atomic activities.
"The US text is not balanced. Washington acts as if nothing positive has happened since November"
Al-Baradai told the IAEA board he was "seriously concerned Iran's October declaration (answering questions about its nuclear programme) did not include any reference to its possession of P-2 centrifuge designs (for making enriched uranium which could be weapon-grade) and related R&D (research and development) which in my view was a setback to Iran's stated policy of transparency."
Al-Baradai said "this was particularly the case" since the October declaration was trumpeted by Iran as providing "the full scope of Iranian nuclear activities" including "a complete centrifuge R&D chronology."
Meanwhile, the IAEA board is expected to report Libya to the UN Security Council for previous non-compliance with the NPT, but praise it for its current cooperation in eliminating weapons development programmes.
A Western diplomat said Libya was "almost done in revealing its nuclear programme" and the non-compliance declaration would merely be a "pro-forma" way of closing the chapter with no threat of sanctions against the North African country.