After talks with EU foreign policy chief Javier Solana and other European officials, Tehran's top nuclear envoy negotiator Ari Larijani warned of a "long road" ahead to solve the crisis.
EU representatives described the meeting as disappointing and unsatisfactory.
The comments came after hardline President Mahmoud Ahmadinejad, speaking in Tehran, promised Iran would continue sensitive nuclear fuel cycle work and would not back down.
"The Iranian nation is determined to obtain all of its rights, including full nuclear rights and the complete exploitation of the nuclear fuel cycle," Mr Ahmadinejad was quoted as saying by the ISNA news agency.
While Iran insists it only wants nuclear power for civilian purposes, the West fears it is covertly trying to build an atomic bomb.
In an effort to get it to freeze uranium enrichment -- a process needed to fuel a nuclear reactor but which could also be used to make a bomb -- the West on June 6 offered Iran a package of economic and political incentives.
The Brussels meeting was aimed at taking stock of developments since then, and to lay the groundwork for talks in France on Wednesday between foreign ministers from the five permanent UN Security Council members plus Germany.
Mr Solana said he would report to the ministers from the six powers which drew up the offer and "we will make an analysis ... to see how we proceed".
Mr Larijani sought to play down the sense of doom, saying the Western offer was broadly acceptable.
"There are different ambiguities but the offer has a central core that is suitable," he said, however added: "We have a long road to travel... We have to be precise and patient."
The US meanwhile underlined that Iran must stop uranium enrichment and reprocessing, the crucial sticking point at the heart of the talks.
And an Iranian official, speaking on condition of anonymity, said: "We agreed on a number of principles but there is disagreement over suspension (of uranium enrichment)."
The six had wanted Iran to respond positively to their offer before a meeting of leaders of the Group of Eight major industrialised countries starting in Saint Petersburg this weekend.
But Ahmadinejad has said Tehran will not respond before August.
The West has made it clear that rejecting the offer would relaunch debate at the UN Security Council on how to further escalate pressure on Tehran.
But it is unclear exactly where this would lead, in particular since Russia and China, which have veto rights, have made it clear they oppose sanctions.