The EU -along with the United States- has engaged Tehran over its determination to enrich uranium, the fuel it wants to use for its nuclear power plants, but which Washington and Brussels say could also be used to make atomic bombs.
"Time is much shorter than the Europeans think," Nasseri told reporters, hinting that Iran would resume uranium enrichment, if the two sides failed to reach an agreement.
"Our point is we simply do not have much time. We have a fuel programme and we can't hold it much further," said Nasseri, a key member of an Iranian delegation headed by Tehran's ambassador to the United Nations, Muhammad Javad Zarif.
European Union negotiators - Britain, Germany and France -are represented at the senior-level talks on Friday by their foreign ministry political directors.
Iran has suspended uranium enrichment as a confidence-building measure for the EU-Iran negotiations that started in December.
"Time is much shorter than the Europeans think"
It is now waiting, however, for an answer from the European trio to a proposal that would allow it to enrich uranium, a process that makes fuel for nuclear power reactors, but, in highly refined form, can be the explosive core of atom bombs.
The European trio is holding fast to its position that Iran must give up on all nuclear fuel activity in order to provide "objective guarantees" that it will not make atomic weapons, diplomats said.
The United States, which backs the EU diplomatic initiative but is not party to the talks, charges that Iran is secretly developing nuclear weapons and must be kept from obtaining the weapons breakout capability, which enrichment represents.
Diplomats have said that the Europeans are only entertaining the Iranian proposal in order to keep the talks going past Iranian presidential elections 17 June, after which the EU hopes Tehran will be more settled politically and possibly be able to make a deal.
But Nasseri said: "We cannot go along with waiting for elections.We need a clear reaction and indication as to how we are going to move forward".
He said Iran still felt "the foundation for an agreement is readily at hand. I believe that both sides are close."
Asked how Iran would react if no agreement was reached on Friday, Nasseri said Tehran would not break off talks. "We are in the process of making a deal, not breaking it," he said.
But he hinted that Iran would either resume uranium enrichment or take steps in that direction, such as preparing uranium for the enrichment process or testing the centrifuge machines that carry out the enrichment, saying: "We can continue negotiations but in a more balanced arrangement. After all, we cannot keep this industry idle for so long."
Iran has already built a uranium
conversion facility at Natanz
It is not clear how the EU would react if Iran resumed enrichment activities; but US officials have said Washington would withdraw its support for the talks and call for Iran to be taken to the UN Security Council for possible sanctions if the enrichment activities halt was broken.
According to the text of the Iranian proposal, read to reporters by a diplomat close to the talks, the Iranians seek the "assembly, installation and testing of 3000 centrifuges in Natanz," the site where Iran wants to build an enrichment plant and has already set up a pilot project of 164 centrifuges.
A sequence, or cascade, of about 2000 centrifuges could make enough highly enriched uranium in a year to make one atom bomb, experts say; although Iran has said it would only make low enriched uranium.
A European diplomat close to the talks said the Iranians would get a "no" to their proposal on Friday, but "camouflaged as well as possible."
"The Europeans will say they are ready to discuss the proposal but not adopt it," the diplomat said.
In The Hague on Thursday, Iran's Foreign Minister Kamal Kharazi said that if there was no agreement in London: "It is our right to restore the programme" of uranium enrichment.
Iran has said repeatedly that its enrichment suspension is temporary and voluntary, as it claims the right to make nuclear fuel under the nuclear Non-Proliferation Treaty (NPT), to which it is a signatory.