The US administration made its expectations clear a day before the release of a crucial report on whether Tehran had complied with UN Security Council demands that it halt sensitive work on uranium enrichment.

Washington has been pressing for possible UN sanctions against the Islamic republic unless it renounced suspected efforts to build a nuclear bomb. But Russia and China have balked at any punitive action.

US Secretary of State Condoleezza Rice raised the stakes for the United Nations on Thursday, telling reporters at a NATO meeting in Sofia that "in order to be credible the Security Council of course has to act."

"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."

Undersecretary of State Nicholas Burns kept up the pressure in Washington, forecasting that Friday's report by International Atomic Energy Agency chief Mohamed ElBaradei would be "strongly negative" for Iran.

"There's no question in my mind that we're going to have to see a significant international response," Burns said. "And that will be one of rebuke of the government of Iran for its actions."

Burns is to meet Tuesday in Paris with political directors of the other four permanent members of the Security Council -- Russia, China, France and Britain -- plus Germany to discuss the next steps with Iran.

US officials said France and Britain could introduce a strong resolution against Tehran soon afterward, and foreign ministers of the "P-5" and Germany could meet in the second week of May.

Washington has pushed the idea of slapping UN sanctions on Tehran's leaders such as a freeze on their assets or travel restrictions, while avoiding any heavy-duty penalties on Iran's oil and gas sector.

But the officials here also appeared increasingly aware of the possibility that the Security Council would not be able to surmount the objections of the veto-wielding Russians and Chinese.

Indeed, Rice's reference to UN credibility carried echoes of the US argument before the Iraq war that the organization had to move against Saddam Hussein's alleged weapons of mass destruction or risk becoming irrelevant.

In recent days, the United States has stepped up its call for other nations to consider action outside the UN framework, such as trade sanctions against Iran or an embargo on sales of arms or technology.

President George W. Bush has also refused to exclude the use of military force. Rice said pointedly last week the United States did not need Security Council approval to assert its "right to self-defense."

But Burns stressed Thursday that "we have not given up hope that there can be a diplomatic solution ... and we're determined to pursue that as aggressively as we can."

The State Department's number three official made his remarks at a joint news conference with Pakistani Foreign Secretary Riaz Khan during a break in a day of strategic talks between their countries.

But if the United States and the mostly Muslim Pakistan are allies in the war on terror, the press session highlighted their differences when it comes to Iran.

Khan stressed that Islamabad was categorically opposed to the use of force against its neighbor and showed little inclination to back eventual coercive diplomatic measures.

"As a neighbor and a country which has very long-standing good relations with Iran, we wish them well," he said.

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