Iran failed to attend a meeting in Vienna to discuss its decision to resume nuclear fuel research, the UN's nuclear watchdog says.
Iran faces the threat of sanctions if it does not halt its nuclear plans
The International Atomic Energy Agency said a meeting with Iranian delegates scheduled for Thursday never took place and no new meeting was planned.
Iran's president earlier insisted its research programme would resume.
The IAEA has warned this may affect talks with the EU aimed at defusing tensions on Iran's nuclear programme.
US Secretary of State Condoleezza Rice also warned that if the negotiations ended, the USA might push for a vote at the UN Security Council on sanctions against Tehran.
Iran suspended the programme two years ago during negotiations with the EU.
But negotiations over the nuclear issue broke down in August, when Tehran announced it would resume converting uranium into gas form.
Correspondents say that resuming nuclear fuel research while the talks are deadlocked risks bringing the negotiations to an end and provoking a fresh crisis between Iran and the IAEA.
The US and EU do not want Iran to be able to manufacture fuel suitable for use in nuclear weapons.
The IAEA's head, Mohamed ElBaradei, said earlier this week he was "seeking clarifications from Iran as to the implications" of Iran's decision.
Mr ElBaradei said Iran should refrain from "all activities linked to uranium enrichment".
But President Mahmoud Ahmadinejad was defiant in a speech in Qom broadcast on Iranian state television on Thursday.
"Some... have said the Iranian nation has no right to nuclear research," he said.
"But they should know that the Iranian nation and government will defend the right to nuclear research and technology and will go forward prudently."
"By relying on its young scientists, Iran will use this technology for medicine, industry and energy in the near future," he added.