They said Britain, France and Germany were now in a minority in calling for a weak resolution at the meeting of the 35-nation board of governors of the International Atomic Energy Agency (IAEA).
The board adjourned at mid-day in order to free diplomats for informal meetings as they tried to hammer out a compromise. The plenary was to reconvene Friday.
But it still was not clear if the United States, which charges that Iran is secretly trying to develop nuclear weapons, will get its wish to declare Tehran in non-compliance with the nuclear Non-Proliferation Treaty (NPT).
This would bring the issue before the United Nations Security Council, which could impose sanctions.
Britain, France and Germany won key concessions of cooperation by Iran with the IAEA when its foreign ministers visited Tehran on October 21, and they fear the threat of sanctions could push Iran to break off cooperation that includes agreeing to snap inspections and suspending the enrichment of uranium.
US President George W. Bush said in London Wednesday that "America believes the IAEA must be true to its purpose and hold Iran to its obligations."
And US Secretary of State Colin Powell on Tuesday described as "just deficient" European readiness to treat Iran's change in attitude since October 21 as a brand new beginning.
Powell complained that a draft resolution by the European big three contained no "trigger mechanisms" setting performance benchmarks to determine whether Iran is to avoid diplomatic or other sanctions.
IAEA chief Mohamed ElBaradei alluded to the conflict when he said Iran must be cited for violating nuclear non-proliferation agreements, but Tehran must also receive credit for turning a new page.
In comments to reporters, he said he expected the board "to respond to the bad news and the good news.
"The bad news is that there have been failures and breaches (in Iranian respect for the NPT). The good news is their cooperation, a new chapter in our cooperation with Iraq," since October 21, he said.
He said he was sure the board would "respond adequately" to a report he filed last week that listed Iranian breaches in compliance during 18 years of hidden nuclear activities, including making small amounts of plutonium and enriched uranium.
But ElBaradei had said in the report there was no proof yet that Iran was secretly trying to make nuclear weapons, something Washington said was "simply impossible to believe."
ElBaradei said the "breaches and failures are, of themselves, a matter of deep concern, and run counter to both the letter and the spirit of safeguards agreements."
He said continued cooperation and "some time and much verification" was needed for the IAEA to clear Iran definitively of charges it has a secret weapons program.
Iran has pledged to sign an additional protocol to the NPT to allow for wider, unannounced inspections. ElBaradei said Iran was already allowing such probes, "acting as if the protocol were in force."
He said the IAEA was also talking to Iran on how to verify Tehran's suspension of uranium enrichment, which the agency had requested.
Tehran has warned that punishing it by taking the issue to the Security Council would set off an international crisis.
The European big three's draft resolution avoids citing Iran for non-compliance.
A revised version, written after US and countries like Australia and Canada protested, had the IAEA saying it "strongly deplores breaches" Iran has made, a diplomat said.
But he said the Americans wanted even tougher wording as they insist on non-compliance being mentioned, as well as the Security Council.
A diplomat said even other European countries had told Britian, France and Germany that their resolution was too weak.