Iran's hardline President Mahmoud Ahmadinejad vowed on Thursday that his country "will not bow to injustice and pressure," over the deadline set by the UN Security Council.
Diplomats said the head of the UN nuclear watchdog, the Vienna-based International Atomic Energy Agency (IAEA), would report on Friday that Iran has failed to meet the UN demands.
And US Secretary of State Condoleezza Rice said the standoff called into question the global body's credibility.
"The United States believes... that in order to be credible the Security
Council of course has to act," Ms Rice told reporters on the sidelines of talks with NATO counterparts in Sofia.
"I sincerely hope that the Security Council is prepared to take some action," she said, adding it was "highly unlikely that Iran will accede to the demands of the international community."
Europeans concerned
France's Foreign Minister Philippe Douste-Blazy also urged the Security Council to send a "rapid and firm" signal to Iran as the UN deadline loomed.
German Foreign Minister Frank-Walter Steinmeier said: "We expect it (Iran) to meet international standards and... to allay world suspicions that its civil nuclear operations are being used to develop a possible weapons program."
Western powers, led by the United States, say Iran is trying to develop a nuclear weapon. Iran insists its research is peaceful.
While the United States has called for tough action, Russia and China have called for more negotiations through the IAEA.
The UN Security Council passed a non-binding statement on March 29, which called on Iran to honour calls by the IAEA to:
• suspend enrichment of uranium and reprocessing work that can make plutonium, a key element in a nuclear bomb.
• "reconsider the construction" of a heavy water reactor that could make large amounts of plutonium.
• grant wider inspections and allow additional "transparency measures".
The Iranian leadership has repeatedly rejected the demands and warned there would be retalliation against any military attack.
Its hardline president reaffirmed the stance on Thursday. "Thanks to God, we are a nuclear state," Mr Ahmadinejad said in a speech.
"We will not bow to injustice and pressure. If they want to attack the rights of the Iranian people, we will stamp shame and regret on them."
Iran's top nuclear official, Vice President Gholam Reza Aghazadeh, held an 11th hour meeting with IAEA director Mohamed ElBaradei on Wednesday. Diplomats said the Iranian official made no new proposal.
UN sanctions
Mr ElBaradei must now make a report to the Security Council. "It is not going to be a good report. That is quite clear. What is there positive to report?" said a senior European diplomat.
Mr Aghazadeh, head of the Atomic Energy Organisation of Iran, "just rattled around on Iran's previously stated positions. He did not propose anything new," said another diplomat about Wednesday's meeting.
Mr ElBaradei's report will be released Friday afternoon "both to member states of the IAEA board of governors and to the UN Security Council," IAEA spokeswoman Melissa Fleming said.
The Security Council faces some hard bargaining over the action it will take however.
The United States has called for a resolution under Chapter Seven of the UN Charter which could allow for sanctions, and eventually military action.
Russia and China, which as permanent members of the Security Council can veto any resolution, are opposed to any hint of sanctions.