The officials told The Associated Press that a brief one-page document Iran presented to European Union foreign policy chief Javier Solana is not, as had been sought, a definitive reply to the offer from major world powers to suspend uranium enrichment and reprocessing in exchange for economic and other benefits.
Instead, the officials said it was a restatement of Tehran's earlier insistence on the right to conduct peaceful nuclear activities and essentially a transcription of portions of recent telephone conversations to that effect between Solana and chief Iranian negotiator Saeed Jalili.
One official familiar with the document described it as "more obfuscation and delay" and not a "real response" to the package, which was presented earlier this year by the five permanent members of the U.N. Security Council and Germany.
The officials spoke on condition of anonymity because Solana's office has not yet characterized the Iranian reply.
The officials said the lack of a clear response meant that discussion of new sanctions against Iran could begin as early as Wednesday when senior diplomats from the six countries that made the offer will speak in a conference call to discuss the way ahead.
"We are looking for a clear, positive response from Iran and in the absence of that we're going to have no choice but to pursue further measures against them," said State Department spokesman Gonzalo Gallegos.
The United States and others accuse Iran of trying to develop nuclear weapons under cover of a civilian atomic energy program. Iran denies the charge.
The EU diplomat said Solana had talked on the telephone with Iran's chief nuclear negotiator, Saeed Jalili, and more talks could be expected in the coming days.
The United States and its European allies fear Iran intends to use the technology to develop material for nuclear weapons under the cloak of a civilian nuclear power program. Iran denies that.
The U.N. Security Council has already adopted three sanctions resolutions against Iran. The United States, the European Union, and individual EU members have imposed their own financial measures against Iranian entities and individuals.
On July 19, the six nations - Britain, China, France, Germany, Russia and the United States - set an informal two-week deadline for Iran to either accept or reject the economic incentives in return for curbing its uranium enrichment.
On Sunday, Iranian President Mahmoud Ahmadinejad said diplomacy was the only way out of the standoff and insisted he was serious about negotiations. Those comments came a day after he asserted his country would not give up its "nuclear rights," signaling that it would refuse demands to stop enriching uranium or at least not to expand its enrichment work.
Also Saturday, Secretary of State Condoleezza Rice said the United States would have no choice "but to begin again to prepare sanctions resolutions for the (U.N.) Security Council" if Iran did not halt the development of its enrichment program.