Iran's suspension of its uranium enrichment programme is not yet complete, the UN's chief nuclear inspector has reported.
Iran denies claims that it wants to build nuclear weapons
Mohamed ElBaradei said Iran still wants to use 20 centrifuges for research. He said he hoped the dispute would be resolved within 24 hours.
Tehran agreed to halt its enrichment programme last week.
The board of the International Atomic Energy Agency (IAEA) is meeting in Vienna to discuss Iran's compliance.
On Wednesday, diplomats said Tehran had asked for an amendment to the terms of last week's deal to allow continued research.
France, Germany and Britain - the three EU countries that helped bring about the suspension - reportedly refused the request.
IAEA inspectors have spent the last few days verifying whether Iran is abiding by the suspension agreement.
Before entering a meeting with the IAEA board on Thursday, Mr ElBaradei said: "I'm going to report that we have completed our work with regard to verification of the suspension with one exception, the request by Iran to exempt 20 centrifuges for research and
development without using nuclear material."
Meanwhile, the EU three are expected to submit a draft IAEA resolution calling on Iran to "sustain the suspension" of uranium enrichment at nuclear facilities in the cities of Isfahan and Natanz.
The motion also proposes that Mr ElBaradei should "report immediately" to the agency's board if there is any evidence of incomplete suspension".
Diplomats who have seen the resolution say it is unlikely to satisfy the US, which is thought to prefer a tougher stance whereby any lapse would immediately trigger Iran's referral to the UN Security Council.
The US has led calls for the IAEA to refer Iran.
The move is strongly opposed by China, another permanent member of the Security Council.
Last week, diplomats said Tehran was rushing through production of uranium hexafluoride gas - a form of uranium that is fed into centrifuges during the enrichment process - before Monday's freeze.
Tehran denounced the accusation as a "sheer lie". It has always maintained its nuclear programme is entirely peaceful.
Two days after the suspension came into effect, diplomats were quoted as saying that Tehran had asked that more than 24 centrifuges be exempted for "research purposes".
Centrifuges purify uranium to fuel power plants or weapons by spinning at supersonic speeds.
"The Iranians asked to be allowed to continue conducting research and development with centrifuges during the freeze, but the Europeans told them no," a Western diplomat told Reuters on condition of anonymity.
The IAEA meeting is also due to discuss South Korea, which has admitted its scientists conducted secret experiments with small quantities of uranium enrichment and plutonium separation.
The IAEA was due to consider whether to refer South Korea to the UN Security Council, though analysts said this was unlikely.