EU foreign policy chief Javier Solana, who had been scheduled to meet Wednesday with Iran's top nuclear negotiator, Ali Larijani, said he was surprised by the move.
"I had made it clear to the Iranians and to Dr. Larijani that we want to proceed rapidly to examine together the ideas I put to him early last month," he said in a statement.
He added that Larijani had assured him they would meet in Brussels on Thursday.
There was no immediate explanation as to what had caused the delay over what could be a crucial round of talks on the package put forward by the United States, Russia, China, Britain, France and Germany.
In Tehran, Iranian Foreign Minister Manouchehr Mottaki confirmed there would be no talks Wednesday, but gave no reason or a new date.
"Today there will be no negotiations between Larijani and Solana," Mottaki told the official Islamic Republic News Agency on the sidelines of a ceremony to welcome the president of Armenia to Iran.
Germany urged Tehran to indicate to the EU whether it would accept the package of incentives.
"We hope to find out whether Iran is prepared to accept the offer," German Foreign Ministry spokesman Martin Jaeger said at a news conference.
The offer, delivered to Tehran by Solana early last month, includes incentives for Iran such as nuclear expertise and reactors if it accepts international oversight of its disputed nuclear program.
Iranian government officials have insisted they need to clear up "ambiguities" contained in the package, and have brushed aside demands that they respond by July 12, when foreign ministers of the five permanent U.N. Security Council nations and Germany consult in Paris.
Jaeger said Tehran had given no indication why Wednesday's meeting was postponed, but played down the importance of the decision to push it back a day.
"I think that what is important is that this meeting takes place," Jaeger said. "What we need, and need quickly, is a concrete signal from Tehran that gives us an indication whether Iran is willing to accept our cooperation offer or how they plan to deal with this offer."
Earlier, EU officials said they did not anticipate Larijani to formally respond to the offer during the talks, but only to seek clarification of several points of the package — and perhaps to come up with a counterproposal.
On Tuesday, Russian President Vladimir Putin urged Iran to accept the package, and British Prime Minister Tony Blair also pushed for a quick reply.
Tehran repeatedly has asserted that its nuclear program, which includes uranium enrichment, is peaceful and aimed at generating power.
But the United States, Israel and the EU fear the research program is a cover for the development of nuclear weapons.
Western diplomats have increased the pressure, saying they would restart efforts to punish Iran through possible Security Council sanctions unless Tehran stops its nuclear activities and agrees to talks by July 12.
Work on a Security Council resolution was suspended May 3 to allow the six powers to draw up the plan of perks if Iran agrees to a long-term moratorium on enrichment — or punishments that include the threat of selective U.N. sanctions if it does not.
Possible U.N.-mandated sanctions include a visa ban on government officials, freezing assets, blocking financial transactions by government figures and those involved in the country's nuclear program, an arms embargo and a blockade on the shipping of refined oil products to Iran.