The talks on Thursday come after Iran disclosed last week the existence of a hitherto secret second uranium enrichment plant and just days after it caused more world anger by testing missiles that could reach Israel.
"Iran is comprehensively failing to cooperate, it is comprehensively failing to live up to its international commitments," German Chancellor Angela Merkel's spokesman Ulrich Wilhelm told reporters.
The UN Security Council has imposed three sets of sanctions against Iran over its refusal to freeze nuclear activities.
The United States and its allies also want Iran to provide UN inspectors immediate access to the newly disclosed uranium plant, which has fueled suspicions that it is seeking the capability to build a nuclear bomb.
The head of the UN's nuclear watchdog, Mohamed ElBaradei, said that Iran was "on the wrong side of the law" by not declaring its new enrichment plant before last week.
"Iran was supposed to inform us on the day it was decided to construct the facility. They have not done that," Mohamed ElBaradei, director of the International Atomic Energy Agency, told Indian TV channel CNN-IBN.
European Union foreign affairs chief Javier Solana will conduct the talks with the top Iranian nuclear negotiator, Saeed Jalili, along with senior officials from Britain, China, France, Germany, Russia and the United States.
Jalili said on Wednesday he was heading for the talks with a "positive approach" while atomic energy chief Ali Akbar Salehi said Iran was ready to discuss concerns about its new enrichment plant.
However, Salehi also maintained there can be no bargaining about Iran's right to master the civilian nuclear fuel cycle under the nuclear Non-Proliferation Treaty and ruled out a freeze on enrichment.
German foreign ministry spokesman Jens Ploetner said the talks were "an opportunity to establish together whether these words, this assurance of a readiness to talk, will be followed by deeds."
Solana stressed ahead of the meeting, the first of its kind for 14 months, that the five permanent powers in the UN Security Council and Germany needed guarantees from Tehran that its nuclear programme was "only peaceful."
Western countries have maintained their aim for a halt of Iran's uranium enrichment in return for a freeze on sanctions, while Russia and China have urged Tehran to cooperate with the IAEA.
"I say to Iran as they face a crucial date this week; join the international community now or face isolation," British Prime Minister Gordon Brown warned.
But hardline President Mahmoud Ahmadinejad lashed out Wednesday at Western demands that Iran give access to the new enrichment site.
"The leaders of these countries made a historic mistake with their comments about the new plant," the state television website quoted Ahmadinejad as saying.
Ahmadinejad said the Geneva talks gave an "exceptional opportunity for US and a few European countries to correct the way they interact with other world nations."
"The negotiators can definitely adopt any policy that they want, but we will not be harmed," the Fars new agency quoted the president as saying.
White House spokesman Robert Gibbs said late Tuesday the United States will mark its return to an active role in the talks by raising its concerns about uranium enrichment, even if Tehran refuses to discuss the issue.
The disclosure of the new plant, being built adjacent to a military base near the central holy city of Qom, has raised suspicion about the nature of Iran's nuclear programme.
The Financial Times reported Wednesday that British intelligence believes Iran has also secretly been designing a nuclear warhead.
French President Nicolas Sarkozy warned last week that new sanctions would be imposed by December if Iran fails to change its stance.
Turkey, a NATO member, said sanctions would be useless. "We do not believe sanctions bear results," Foreign Minister Ahmet Davutoglu said Wednesday. "The problems must be resolved through diplomacy only."
