The UN's nuclear watchdog is to report to the Security Council on Friday on whether Iran has complied with demands to suspend uranium enrichment.
Iran insists it will not abandon its right to enrich nuclear fuel for what it says are wholly peaceful purposes. But the US has urged the UN to take action if Iran is found not to have complied with its demands.
The US secretary of state says the Security Council credibility is on the line over its handling of the issue.
The US fears Iran will be use the fuel to build a nuclear bomb and has not ruled out armed action against Iran.
Washington is trying to rally support from the Security Council for tougher action against the Tehran government, including sanctions.
'Worrying situation'
Speaking at a meeting of Nato foreign ministers in the Bulgarian capital, Sofia, US Secretary of State Condoleezza Rice said the Security Council's credibility will be tested over how it dealt with Iran.
"The Security Council is the primary and most important institution for the maintenance of peace and stability and security and it cannot have its word and its will simply ignored by a member state," she said.
Her call was backed by French Foreign Minister Philippe Douste-Blazy, who said the world had to show "unity and firmness" towards Iran.
"The situation is serious and worrying," he said, "there is nothing to suggest that Iran is conforming to the demands of the international community."
Defiant speech
The UN Security Council demanded on 29 March that Iran comply with the demands of the IAEA for a "full and sustained suspension" of its uranium enrichment work.
Mohammed ElBaradei, the head of the UN's nuclear watchdog, the IAEA, will report back on Friday on whether Iran has complied with those demands.
Iran says it has the right to peaceful nuclear technology and denies Western claims that it is seeking weapons.
Its leaders have repeatedly said they will ignore foreign pressure to abandon their nuclear programme.
Addressing a crowd in Zanjan in north-west Iran on Thursday, President Mahmoud Ahmadinejad said Iran would not give in one iota to "oppression".
"We have obtained the technology for producing nuclear fuel... no-one can take it away from our nation," he said.
"If you think by frowning at us, by issuing resolutions... you can impose anything on the Iranian nation or force it to abandon its obvious right, you still don't know its power," he said, his speech carried live on state television.
Calls for restraint
China and Russia, who are both veto-holding members of the Security Council, are opposed to sanctions and have urged restraint from all sides.
Russian President Vladimir Putin said the IAEA needed to continue to play a key role in the crisis.
"We believe it is the IAEA that must play a key role and not have this weight unloaded on to the back of the Security Council," Mr Putin said.
In Beijing, Chinese foreign ministry spokesman Qin Gang issued an appeal for calm.
"We hope the relevant parties can keep calm and exercise restraint so as to avoid moves that would further escalate the situation," Mr Qin said.
He said the problem could still be "resolved through dialogue and diplomatic means, which is the correct choice for all parties concerned".
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