The talks, considered crucial, are aimed at avoiding an escalation of Tehran's standoff with the West on its nuclear programme after Iran warned there was a high risk of deadlock.
"The talks between (Iranian and European) experts have been difficult and complicated, they haven't been promising and if they go on like this, the risk of a deadlock in the negotiations Wednesday is high," Ali Agha Mohammadi, one of the Iranian negotiators currently in Geneva, said.
He was speaking to AFP after official-level talks in Brussels on Tuesday to prepare for the formal negotiations in Geneva between the foreign ministers of Britain, France and Germany and Rowhani.
Iran has described the meeting in the Swiss city as a "last-ditch meeting" to avoid referring Iran to the UN Security Council - and into Washington's diplomatic line of fire - if the talks fail.
"It is our only hope that the three European ministers who proposed this meeting will try to lead the negotiations out of this situation so we can make reasonable progress," Mohammadi said on Tuesday.
"In spite of this situation we will take part in tomorrow's meeting because the meeting was proposed by the three European ministers and we are the guests of these negotiations," he added.
Isfahan, south of Tehran, is one
of the uranium-processing sites
The 25-nation EU has warned that it could refer Iran to the UN Security Council if the talks fail.
"Nobody wants a crisis on our side. We want the talks to continue," one EU diplomat said earlier, in advance of Wednesday's meeting, due to start at 2pm (1200 GMT), which will also involve EU foreign policy chief Javier Solana.
The so-called EU-3, representing the full 25-member EU, called the talks after a series of recent threats from Tehran to resume key nuclear activities, in breach of an accord to suspend them last November.
In contrast to the US, which suspects Tehran of wanting to build nuclear bombs, the EU-3 is seeking to engage the Islamic state, using a carrot of possible trade and other benefits to persuade it to curb its nuclear plans.
Nevertheless a top US official backed the EU efforts.
"The US supports the EU-3's negotiating efforts to end Iran's nuclear weapons ambitions. Iran must not be allowed to acquire a nuclear weapons capability," US Under Secretary of State Nicholas Burns said.
"Iran has an obligation to demonstrate to the world it is not doing so."
Iran wants the Europeans to help
them fabricate nuclear reactors
In a letter to Rowhani, calling for the talks, the EU-3 said that "Iran should be in no doubt that any such change to the suspension would be a clear breach of the Paris agreement" of last November.
"It would bring the negotiating process to an end. The consequences beyond could only be negative for Iran," added the letter, sent on 13 May and a copy of which was obtained by AFP.
Iran has warned bluntly that the talks are the "last chance" for the Europeans to offer it enough of an incentive to stop it resuming uranium enrichment activities, as threatened.
Quid pro quo
Enriched uranium can be used for civilian as well as military purposes.
According to the Washington-based Carnegie Endowment for International Peace, Tehran is notably planning to partially resume activities at a plant in Isfahan, central Iran, followed by another site, Natanz.
In return Iran would pledge not to acquire nuclear arms and would authorise the permanent presence of IAEA inspectors at Isfahan and Natanz, according to documents obtained by the Carnegie Endowment and confirmed by Iranian sources.
The Iranians also want the EU to help them build nuclear reactors, and to guarantee them supplies of nuclear fuel for future reactors.