An International Atomic Energy Agency report expresses worry that Iran may be working on a nuclear warhead, despite a 2007 U.S. intelligence assessment that found the Islamic Republic stopped such work in 2003.
The United Nations' nuclear watchdog also confirmed that Iran had indeed enriched uranium to nearly 20 percent, a claim made by President Mahmoud Ahmadinejad during revolutionary anniversary festivities last week but rebuffed by the White House.
"We do not believe they have the capability to enrich to the degree they say they are enriching," White House Press Secretary Robert Gibbs said at last Thursday's daily briefing.
But the IAEA report said that Iran had hit 19.8 percent enrichment on two days last week.
"I think the conclusions of the report are consistent with what the secretary was saying in the region this week," State Department spokesman P.J. Crowley said Thursday. "We have ongoing concerns about Iran's activities. We cannot explain why it refuses to come to the table and engage constructively to answer the questions that have been raised, and you have to draw some conclusions from that."
The report by the new head of the IAEA, Yukiya Amano, appears to raise greater concerns about Iran's capabilities than the assessments of his predecessor, Mohamed ElBaradei.
"Altogether, this raises concerns about the possible existence in Iran of past or current undisclosed activities related to the development of a nuclear payload for a missile," said the report.
"These alleged activities consist of a number of projects and sub-projects, covering nuclear and missile related aspects, run by military related organizations."
The Associated Press indicated that the U.S. may go back and revise the intelligence assessment to determine if Iran resumed weapons work after the 2007 report.
In Tehran on Wednesday, Ahmadinejad said "the U.S. supremacy is melting away just like snow in front of the sun," according to the semi-official Fars News Agency.