One reason is Tehran's insistence of its right under the international Nuclear Non-Proliferation Treaty (NPT) to produce its own reactor fuel -- a right that it says it might briefly suspend but will never give up.
Hossein Mousavian, Iran's chief delegate to the UN's International Atomic Energy Agency (IAEA), put Tehran's position this way: "We have emphasized that the suspension [of uranium-enrichment activities] should be for confidence-building, not as a legal obligation."
Analysts say statements like those make Washington skeptical that Iran and the three EU states can reach a long-term accord that satisfies all sides. A similar "suspension" deal between European powers and Iran in late 2003 fell apart amid disagreements over the terms.
Iran has already come under criticism this week by diplomats for breaking the spirit of its nuclear accord with the EU by using a loophole to keep preparing raw uranium for nuclear enrichment.
David Albright, a nuclear expert at the Institute for Science and International Security in Washington, said U.S. officials are watching the EU deal with interest because -- if successful -- it could be an ideal solution to the crisis.
"I think many in the U.S. government want [the European effort] to succeed," Albright said. "It's a dream deal in terms of U.S. objectives to get Iran to give up its ability to make nuclear explosives material and have that verified, and then have Iran shift its civil nuclear energy program toward just nuclear electricity production using imported reactors."
Earlier this week, U.S. President George W. Bush reiterated Washington's desire to see the nuclear disputes with both Iran and North Korea resolved through talks.
"Diplomacy must be the first choice and always the first choice of an administration trying to solve an issue of, in this case, nuclear armament, and we'll continue to press on diplomacy," Bush said.
But Albright said U.S. officials do not really believe Iran is ready to give up what Washington says has been a determined effort to acquire nuclear weapons. Iran has denied such allegations, saying it needs nuclear power stations to meet domestic energy demands.
Analysts said that means that, over the coming months, the Iranian nuclear crisis could go in either of two directions.
One possibility is that the European initiative will lead to good-faith negotiations with Iran. Then, the United States would have to decide whether to abandon its skepticism and join the talks in an effort to reach a final "grand bargain" that would end the nuclear crisis.
Neil Partrick of the Economist Intelligence Unit in London said Washington's participation would be necessary because Iran would most likely want incentives from the United States, too, as part of any final deal.
"The Iranian version of a grand bargain -- as far as it's possible to divine a clear line on this -- would be one that involves a significant degree of engagement by the U.S.," Partrick said. "And the Europeans must be seen as really rather secondary players on this issue, ultimately. And along with that engagement would come some [demands for] clear guarantees about [Iran's] own security."
But Partrick said hostile relations between the United States and Iran could make it difficult for any American administration to join the EU nations in negotiating directly with Tehran.
"It's very hard to imagine a U.S. administration of any kind being prepared to make those kind of guarantees to an Iranian regime that remains extremely controversial [in America]," Partrick said.
Washington has had no formal relations with Iran since U.S. diplomats were taken hostage for 444 days in Tehran immediately after the 1979 Islamic Revolution.
The second possible course for the nuclear crisis is that Tehran could balk at abandoning its uranium-enrichment activities. Then, a frustrated Europe might move closer to Washington's position -- that is, that Iran must be forced to do so.
In that case, Washington would likely try to enlist the Europeans in its own efforts to persuade the International Atomic Energy Agency to refer Tehran to the UN Security Council for discussion of possible sanctions. The United States might also seek to persuade the European Union to join it in its efforts to isolate Iran politically or economically.
With so many variables at play, analysts said it is impossible to predict how the Iranian nuclear crisis might end. But many said the least likely scenario at the moment is U.S. military action against Iran.
Albright called U.S. air strikes a "poor option," precisely because Washington's greatest worry about Iran is that it could be pursuing weapons development work at sites that have not yet been discovered. He said that means Washington could never be sure its air strikes had destroyed all of any clandestine nuclear program.
And such strikes would raise a new problem of how to deal with an Iranian government that would be only more convinced it needs nuclear weapons for its own security.