EU foreign policy chief Javier Solana has said he is disappointed after talks with Iran on its nuclear programme, as he prepares a report for the UN.
Iran says its programme is for purely peaceful purposes
But top Iranian nuclear negotiator Saeed Jalili said the discussions were positive and more would be held.
The US and its allies are now expected to press for more sanctions on Tehran, but Mr Jalili said such pressure would not make Iran curb its nuclear plans.
The UN is demanding that Iran suspend uranium enrichment.
Iran says its programme is peaceful, but some Western powers fear it is seeking to make weapons.
'I expected more'
The two envoys emerged after five hours of talks and spoke to the media separately, giving dramatically different interpretations of the outcome.
Talks between Jalili (l) and Solana have been frequently delayed
BBC diplomatic correspondent Jonathan Marcus says Mr Solana was looking for signs that the Iranians were prepared to suspend uranium enrichment work as demanded by the UN Security Council, but his comments after the talks suggest there has been no progress on the issue.
"I expected more and am therefore disappointed," the EU official said.
"I will be in telephonic contact with the Iranians before the end of December," he added, saying the envoys would meet again if circumstances permitted.
Meanwhile Mr Jalili said the two sides had agreed to continue negotiations and arrange another meeting next month.
But he said it was unacceptable to pressure Iran into abandoning uranium enrichment, as it was allowed to do so under the nuclear Non-Proliferation Treaty.
"It is unacceptable that Iran should be deprived of its rights when it has fulfilled its duties," he said, quoted by AFP news agency.
Further UN sanctions would not work, he added.
"Iran has removed concerns and cooperated with the IAEA [International Atomic Energy Agency]. But if some countries want to use the UN Security Council and its resolutions to stop Iran's atomic work, surely they will not be successful," he said.
The UN had earlier commissioned two reports on Iran's nuclear programme - one from the International Atomic Energy Agency (IAEA) and the other from Mr Solana.
Ahead of the talks, Iranian government spokesman Gholam Hossein Elham did say that Mr Jalili would "present new ideas and initiatives" to Mr Solana.
However, on Wednesday, President Mahmoud Ahmadinejad insisted Iran was "a nuclear nation", adding: "After this, no-one can threaten the Iranian nation as we have all stood united so far and [the West] did not do anything."
The BBC's Pam O'Toole says there is a distinct air of pessimism around the latest talks, given that Iran has on a number of occasions announced proposals and suggested initiatives while continuing to defy the UN on uranium enrichment.
Mr Solana has himself appeared frustrated that his talks with Mr Jalili have been frequently delayed.
Mr Jalili, a close ally of Mr Ahmadinejad, recently replaced Ali Larijani, who resigned as chief nuclear negotiator amid reports of differences with the leadership.