A senior official of the United Nations nuclear supervisory body said on Thursday that talks a day earlier in Iran had ended inconclusively and international inspectors had not been given access to a site that they suspect may have been used for testing bomb triggers, The New York Times reported.
Herman Nackaerts, the deputy director of the International Atomic Energy Agency, said the discussions "could not finalize" a document that "once agreed, should facilitate the resolution of outstanding issues regarding possible military dimensions of Iran's nuclear program."
He declined to say whether any progress had been made.
The talks have been going on for months, veering from apparent optimism last May when Yukiya Amano, the I.A.E.A. director general, said there had been a decision "to conclude and sign" an agreement, to far more muted recent assessments. Before Wednesday's talks, Mr. Amano said: "The outlook is not bright."
Mr. Nackaerts was speaking as he returned from Tehran to Vienna, where the I.A.E.A. has its headquarters. He said no date had been set for further talks.
"We will work hard now to try and resolve the remaining differences, but time is needed to reflect on the way forward," he said.
"As on previous occasions, we were not granted access to Parchin," he said, referring to a military site 20 miles south of Tehran that inspectors want to examine for evidence of bomb trigger tests.
While Western powers suspect Iran is seeking nuclear weapons technology, Tehran says its program is for civilian purposes.
The Tehran talks coincided with conflicting signals suggesting that Iran is pressing ahead with the modernization of part of its program to enrich uranium to less than 5 percent purity while potentially slowing the expansion of a stockpile of more highly enriched uranium.
Iran's top Iranian atomic energy official was quoted on Wednesday as saying that his country had begun to install more sophisticated enrichment devices at its Natanz nuclear site.
Fereydoon Abbasi, the head of Iran's Atomic Energy Agency, said scientists began putting in new centrifuges last month, the semiofficial Iranian Students' News Agency said. "We have produced the machines as planned, and we are carrying out the installation gradually, to complete the tests relevant to the new generation," he was quoted as saying.
Iran told the I.A.E.A. late last month that it planned to install the new equipment at Natanz, southeast of Tehran, to speed up the production of enriched uranium, a move that seemed likely to worry the United States, Israel and the West.
But another Iranian news agency, Fars, quoted Mr. Abbasi as saying that the new centrifuges were designed to enrich uranium to a purity of less than 5 percent, not to the 20 percent level that prompts concerns about possible use in nuclear weapons.
On Tuesday, Iran said that it was converting some of its stockpile of 20 percent enriched uranium into reactor fuel. Diplomats in Vienna said that once that is done, it is difficult to reconvert it for weapons.
Some analysts argue that, by slowing the growth of its stockpile, Tehran could be delaying the moment when it reaches a size large enough to prompt possible military action by Israel, which has signaled readiness to attack Iran's nuclear sites pre-emptively. Iran denies that it is seeking the wherewithal to build a nuclear weapon.
On a separate negotiating track, Iranian officials are scheduled to meet in Kazakhstan this month with representatives of six powers - the United States, China, Russia, Britain, France and Germany - for the next round in a long-running series of talks about curbing Iran's uranium enrichment program. So far those talks have also been inconclusive.
Iran came under new pressure from President Obama in his State of the Union address to settle its differences with the West.
"The leaders of Iran must recognize that now is the time for a diplomatic solution, because a coalition stands united in demanding that they meet their obligations, and we will do what is necessary to prevent them from getting a nuclear weapon," Mr. Obama said.