The United States said Monday Iran needed to start answering questions raised in a UN report that states Tehran may be working on a nuclear warhead, rather than criticize the agency.

"It would be better for Iran, rather than protesting this report, to come forward in a constructive way and answer the questions that the IAEA has on behalf of the international community as a whole," State Department spokesman Philip Crowley told reporters.

Iran's supreme leader Ayatollah Ali Khamenei said Sunday the UN nuclear watchdog lacks independence -- singling out the United States for influencing the body -- ahead of the 35-member meeting this week in Vienna to discuss the new report on Tehran's atomic program.

"This is about questions that the world at large has about Iran, the role it's playing in the region, and its nuclear concerns," Crowley said.

The IAEA's work "represents not just the concerns the United States has, but the concerns the international community has," he said.

The IAEA's most recent report expresses concern at Tehran's nuclear program and says the Islamic republic may "currently" be working on a nuclear warhead.

The report also confirms that Iran has started enriching uranium to higher levels, theoretically bringing it close to the levels needed for an atomic bomb.

US President Barack Obama reached out to Iran after taking office last year, offering dialogue to help resolve three decades of animosity between the two nations.

But his administration's concerns have been growing about Iran's nuclear activities and human rights record.

Secretary of State Hillary Clinton said Monday said it could take months for new UN sanctions against Iran, as she prepared for talks in Argentina and Brazil about the perceived Iranian nuclear threat.

Britain, France and Germany support tougher sanctions against Iran. But China is hesitant and Russia, while critical of Tehran's behavior, says it opposes the type of "crippling" sanctions sought by Israel.