Washington is specifically seeking a declaration from the board of the International Atomic Energy Agency that Iran has violated the Nuclear Nonproliferation Treaty, which it has signed, said the diplomats.
The United States has accused Iran of secretly embarking on a program to enrich uranium at Natanz in southern Iran, which it fears could be used to make nuclear weapons.
They said U.S. requests for support have gone out to Russia, France, Britain, Germany and other members of the 35-nation board — the key decision-maker at the U.N. nuclear watchdog agency — ahead of its meeting next month.
Depending on its language, such a declaration could restrict itself to expressing concern about such a violation or increase pressure on Tehran to account for its activities by referring the issue to the Security Council.
The U.S. succeeded earlier this year in getting the IAEA to refer North Korea's nuclear program to the Security Council.
In Iran's case, such a move would further strain already burdened U.S.-Iranian relations that took a turn for the worse last year, after President Bush labeled Tehran part of the "axis of evil" for its alleged support of terrorism.
More recently, Tehran has said it would not recognize any U.S.-installed government in Baghdad. And Washington signed a truce with the People's Mujahedeen, which opposes Tehran's cleric-dominated government, allowing it to keep its weapons although the Iraqi-based group is on the U.S. State Department's terrorist list.
The nature of work at the Natanz site was not known until last year, and the diplomats, who spoke separately and on condition of anonymity, said Mohamed ElBaradei, director-general of the Vienna-based IAEA, was taken aback at what he saw on a visit to the facility in February.
"It's a sophisticated uranium-enrichment plant, and they had come a long way," said one diplomat familiar with the findings of the visit and the workings of the agency. "He was struck by the sophistication and the advanced stage of the project."
The diplomat said U.S. officials "want the agency to produce a very critical report" at the board meeting.
Agency officials said it was too early to comment on the Iranian nuclear program and whether Tehran had violated its Nonproliferation Treaty commitments.
"We are at the moment in the process of conducting inspections in Iran and of doing analysis at IAEA headquarters, and at this point we are reserving judgment about the nature of Iran's nuclear program," said spokeswoman Melissa Fleming.
Members of the U.S. delegation to the IAEA declined comment. But a senior Western diplomat familiar with the issue said other capitals would likely be receptive to U.S. overtures for support at the board meeting.
He said he did not expect French, German and Russian displeasure over the U.S.-led invasion of Iraq to blunt international concern about Iran's nuclear programs.
With Russia's nuclear cooperation with Iran also being criticized by Washington, however, it was unclear how willing Moscow would be to back a tough resolution.
The United States claims that the technology and expertise Iran is gaining from Russia's construction of the $800 million Bushehr nuclear power plant could be used for a weapons program, and that Russian companies — perhaps without official permission — have transferred weapons technology to Tehran.
Senior Russian officials earlier this week said there was no evidence Iran was pursuing a nuclear weapons, while acknowledging that Tehran had to show more transparency in its nuclear programs.
In a CIA report issued last year, the U.S. also said Chinese assistance with civilian industries was increasing Tehran's ability to develop weapons.
Seeking to counter the U.S. push, a top Iranian official Tuesday denied his country had a nuclear weapons program but told the IAEA his country would not automatically submit to tougher inspections of its facilities.
Iran's atomic energy chief, Gholamreza Aghazadeh, told the agency that Iran's nuclear program was only for peaceful purposes, said a diplomat at a closed-door meeting attended by representatives of 135 member nations.
Iranian officials have said they have nothing to hide because their nuclear program is only meant to generate electricity. But U.S. officials scoff at that claim, contending that oil-rich Iran has little need for nuclear power.