The signs of tension came as the UN watchdog International Atomic Energy Agency (IAEA) prepared to release a report at the end of a 30-day deadline for Tehran to comply with UN demands to suspend uranium enrichment.

US Secretary of State Condoleezza Rice used a meeting with her NATO counterparts in Sofia to urge firm action against the Islamic regime, which Washington accuses of seeking to develop nuclear arms.

"In order to be credible the Security Council of course has to act," Rice told reporters on the sidelines of the talks which wrapped up in Sofia on Friday.

Concern over Tehran's intentions was fueled Friday when President Mahmoud Ahmadinejad was quoted as saying that Iran could soon become a world superpower.

"The Iranian nation's achievement of peaceful nuclear energy is so important that it could change the world equation," Ahmadinejad was quoted by official media as saying during a visit to the northern province of Zanjan.

German Foreign Minister Frank-Walter Steinmeier said diplomatic efforts "will have to be maintained and strengthened... to show the unity of the international community". "The reactions from Tehran are cause for concern.... All NATO states agree that this is the only way to make the consequences of its self-isolation sufficiently clear to Iran," he added.

But Russian Foreign Minister Sergei Lavrov took a markedly different line from the United States, underlining above all the need for UN inspectors to remain in Iran to ensure compliance with the nuclear Non-Proliferation Treaty.

"Russia continues to be concerned to enable the IAEA to continue its efforts in Iran. Only if professional inspectors can continue to work can we ensure that the NPT is complied with in respect of nuclear weapons," he said.

While couched in diplomatic language, the call was seen by some as a warning against overhasty action against Tehran, since the imposition of sanctions by the UN Security Council could threaten the IAEA inspectors' position.

"We think that in the future all of our efforts should be directed towards not allowing the operations of the IAEA in Iran to be undermined in any way," said Lavrov.

Later Friday the head of the UN nuclear watchdog Mohamed ElBaradei was expected to report that Iran has failed to meet a UN deadline to stop enriching uranium, opening the door to possible sanctions against the Islamic republic.

Underlining transatlantic efforts to speak with one voice, France's Foreign Minister Philippe Douste-Blazy echoed the US call for the Security Council to send a "rapid and firm" signal to Iran.

"The situation is serious and worrying," Douste-Blazy said, adding: "There is nothing to suggest that Iran is conforming to the demands of the international community."

Western powers, led by the United States, allege Iran is trying to develop a nuclear weapons capability but Iran insists it is developing nuclear technology only to produce atomic energy.

However, both Russia and China -- trade partners with Iran and permanent members of the Security Council with veto power -- have rebuffed Washington's call for possible sanctions and have pressed for continued negotiation.

While not formally on the NATO agenda, Iran was on the menu at a so-called "transatlantic dinner" Thursday evening which gathered the NATO ministers with their European Union colleagues and EU foreign policy chief Javier Solana.

Officials were tightlipped afterwards. "There was a sense of commitment to unity," said one US official, declining to comment further.

"The United States restated that it remains firmly committed to the diplomatic course," said one diplomat.

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