Iran has threatened to end UN inspections of its nuclear sites and resume uranium enrichment if it is reported to the UN Security Council as agreed by the council's five permanent members.
In a confidential new report, the UN nuclear watchdog said Iran had already begun preparing for uranium enrichment, which can potentially produce fuel for bombs, and continued to hinder inquiries into its atomic activities.
The big powers said a crisis meeting of the International Atomic Energy Agency (IAEA) board on Thursday should "report to the Security Council on the steps required from Iran".
But they said the council should then wait until IAEA chief Mohamed ElBaradei reports on Iran's nuclear programme at a regular IAEA meeting on March 6 before deciding on any action.
That gives Tehran a few more weeks to try to negotiate its way out of the crisis, though it gave little sign of compromise.
"We will never abandon our rights to nuclear technology and if referred to the Security Council, Iran will stop voluntary implementation of the Additional Protocol," Iranian President Mahmoud Ahmadinejad was quoted as saying by the semi-official Mehr news agency.
The nuclear Non-Proliferation Treaty's Additional Protocol, signed but not ratified by Iran, gives UN inspectors greater powers of access to suspected sites, including spot checks.
Mehr also quoted chief nuclear negotiator Ali Larijani as saying Tehran would lift all suspensions on nuclear work if referred or reported to the Security Council.
A joint statement issued after a late-night meeting in London of the foreign ministers of the big five plus Germany and the European Union urged Iran to restore a moratorium on all uranium enrichment-related activity, under IAEA supervision.
Iran, which says its nuclear programme is purely peaceful, announced this month it was resuming nuclear research which had been suspended for 2-1/2 years.