European nations have called on Iran to reverse its decision on Tuesday to resume nuclear fuel research, part of its controversial nuclear programme.
Iran faces the threat of sanctions if it does not halt its nuclear plans
Germany said it viewed the announcement with "concern" and said it could "throw into doubt the exploratory talks" scheduled to be held on 18 January.
France said Iran's decision was "very worrying" and would violate UN demands.
The US and EU do not want Iran to produce fuel suitable for use in nuclear bombs.
"We strongly urge Iran to go back on this announcement which, if it were implemented, would clearly run counter to the demands of the International Atomic Energy Agency's (IAEA) board of governors," French Foreign Ministry spokesman Jean-Baptiste Mattei told reporters.
"Full doubts remain about the goals of Iran's nuclear program."
German foreign ministry spokesman Martin Jaeger echoed Mr Mattei's comments.
"We view the latest announcement from Iran that it will restart its research and development work with concern," he told a press conference.
The two members of the EU3 negotiating team, which also includes the UK, were reacting to comments made on Tuesday by Iran's chief nuclear negotiator, Ali Larijani.
Mr Larijani warned Europe not to push Iran towards its 'second scenario'
Talking to Iranian Jam-e Jam TV, Mr Larijani said Iran's nuclear fuel research was "not negotiable".
"The suspension of Iran's nuclear research was a mistake and an irrational decision," Mr Larijani said.
Iran suspended research in November 2003.
"Nuclear research is not related to industrial production and there is no need for others to be concerned about Iran's nuclear research," he added.
IRAN'S NUCLEAR STANDOFF
September 2002: Work begins on Iran's first nuclear reactor at Bushehr
December 2002: Satellite photographs reveal nuclear sites at Arak and Natanz. Iran agrees to an IAEA inspection
September 2003: IAEA gives Iran weeks to prove it is not pursuing atomic weapons
November 2003: Iran suspends uranium enrichment and allows tougher inspections; IAEA says no proof of any weapons programme
June 2004: IAEA rebukes Iran for not fully co-operating with nuclear inquiry
November 2004: Iran suspends uranium enrichment as part of deal with EU
August 2005: Iran rejects EU proposals and resumes work at Isfahan nuclear plant
January 2006: Iran announces it will resume nuclear fuel research under the supervision of the IAEA.
"If the West is concerned about Iran's deviation towards developing nuclear installations, it can be certain that no deviation will occur from nuclear research."
Iran says the research will be supervised by the IAEA.
Mr Larijani also denied Iran was trying to seize the initiative before the next stage of talks with the EU3 on Iran's nuclear programme scheduled to be held in Vienna at the end of the month.
"We are not pessimistic about the talks, but at the same time, we warn Europe not to push Iran towards the 'second scenario'," he said.
But Mr Larijani did not elaborate on what Iran's 'second scenario' was.
"Our scenarios are planned in such a way that, if we lose, others in the region will lose too."