Iran has asked for an amendment to the terms of its freeze on uranium enrichment activities to enable it to continue research, diplomats say.
Iran denies claims that it wants to build nuclear weapons
Iran said on Monday it was halting enrichment as part of an agreement with the EU aimed at avoiding UN sanctions.
Diplomats say Tehran's request for a small quantity of technical equipment to be exempted from the deal has been refused by the EU.
The UN nuclear agency meets on Thursday to discuss Iran's nuclear ambitions.
Head of the International Atomic Energy Agency (IAEA), Mohamed ElBaradei, will tell the meeting whether he thinks Iran is abiding by the suspension agreement.
Western diplomats said on Wednesday that Tehran had asked that more than 24 centrifuges be exempt for "research purposes".
Centrifuges purify uranium to fuel power plants or weapons by spinning at supersonic speeds, Reuters news agency reports.
"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.
Last week, diplomats said Tehran was rushing through production of uranium hexafluoride (UF6) gas - a form of uranium that is fed into centrifuges during the enrichment process - ahead of Monday's freeze.
Tehran denounced the accusation as a "sheer lie". It has always maintained its nuclear programme is entirely peaceful.
France, Britain and Germany has drawn up a draft IAEA resolution calling on Iran to "sustain the suspension" of uranium enrichment at nuclear facilities in the cities of Isfahan and Natanz.
It is to be submitted to the IAEA board meeting on Thursday.
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 to the Security Council, which has the power to impose sanctions.
Meanwhile, a CIA report to the US Congress says Iran was helped with its nuclear programme by the network headed by disgraced Pakistani nuclear scientist Abdul Qadeer Khan.
The CIA said Iran was provided with designs for Pakistan's older centrifuges and for more advanced and efficient models and components before the second half of 2003 - the period covered in the report, AFP news agency reported.
Dr Khan publicly admitted last February his involvement in the illegal transfer of nuclear secrets to Iran, Libya and North Korea.