"We have got agreement on an offer that will be made to the government of Iran," British Foreign Secretary David Miliband announced, after discussions with counterparts from China, France, Germany, Russia and the United States.
In a brief statement, host Miliband explained that the powers had "reviewed and updated" an offer made to Iran in June 2006, but that the contents of the new proposal would only be disclosed to the Islamic republic.
The six have been using a twin-track, carrot-and-stick approach of incentives mixed with sanctions.
Miliband said: "We will be transmitting that offer, we won't be revealing details except to the government of Iran and we very much hope that they will recognise the seriousness and the severity with which we have approached this issue and that they will respond in a timely manner to the suggestions we are making."
The five permanent members of the United Nations Security Council plus Germany have been seeking to persuade Iran to rein in its nuclear work.
Western powers fear Tehran wants to use its nuclear programme to make atomic weapons but the Islamic republic insists the drive is peaceful and solely aimed at providing energy for a growing population.
Miliband said the proposal was designed to show Tehran "the benefits of cooperating with the international community," which had a "grave problem" over its nuclear ambitions.
"We are united in our belief that the threat posed by this enrichment programme to stability is very serious and it's one that we want to address directly.
"Iran says that it wants to play a constructive role. We believe that the rights that it seeks need to be accompanied by a clear set of responsibilities."
French Foreign Minister Bernard Kouchner cast some light on the new elements.
"It was propositions which were not on the table," he told reporters.
"These are propositions that the group, that's to say the six, applying the United Nations resolutions, are offering in a very precise and very detailed way on the various chapters. That had never been done."
A senior US State Department official told reporters that the six powers "went through the refreshed paper" which updates the 2006 offer, but declined to say whether it contained new proposals.
"I expect within a week we will have finalised everything and I think we will have something ready" to present to Tehran, he added, speaking on condition of anonymity.
Meanwhile a European diplomatic source said the document was "an upgrade along this line of making this substantial offer more attractive."
The six powers have offered technical, political and economic rewards to Tehran for suspending its nuclear programme.
At the same time, the UN Security Council has adopted three resolutions imposing sanctions on Iran for its refusal to suspend its uranium enrichment activities.
