In a brief statement, Miliband explained that the six nations 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 Tehran.

Alongside the incentives, the UN Security Council has adopted three resolutions imposing sanctions on Iran.

Enriched uranium can be used either to produce nuclear power or in weapons.

Miliband said that the proposal was designed to show Tehran "the benefits of co-operating with the international community," which had a "grave problem" with its nuclear ambitions.

"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," Miliband said.

Iran turned down the previous offer has repeatedly insisted that it has the right to a nuclear programme to help it meet its energy needs. 

Miliband set Tehran a deadline to respond or say what the group would do if Iran rejects the package again.