Iran has just sent a letter to the U.N.'s nuclear watchdog, the International Atomic Energy Agency, repeating its request for fuel to run a research reactor in Tehran that produces nuclear isotopes for medical purposes, according to U.S. and European officials.

They are reading the letter as Iran's latest, and perhaps final, rejection of an offer the United States and five other countries made last October to provide the fuel by taking low-enriched uranium out of Iran and enriching it for use in the research reactor. (The plan would have significantly reduced Iran's low-enriched uranium stockpile available for enriching into weapons-grade fissile material).

News of Iran's letter comes a day after the IAEA said for the first time in a report that it suspects Iran may be trying to develop a nuclear warhead that could fit inside the nose-cone of a ballistic missile.

"We understand that Iran has recently sent a letter to the IAEA that simply repeats its request from last year for assistance to acquire fuel - a request the IAEA has responded appropriately to with its offer last October," National Security Council spokesman Michael Hammer said.

"We see nothing new, and it would appear to reiterate Iran's rejection of the IAEA's proposal. Coupled with the IAEA's latest report on Iran's nuclear program, this reinforces why our concerns about Iran's nuclear intentions are deepening," Hammer said.

N&S doesn't have the full text of the Iranian letter yet, but stay tuned.

One line of thought here is that Iran may be setting the stage for a deeper confrontation with the IAEA. Under the nuclear Non-Proliferation Treaty, the international community, through the IAEA, is supposed to help non-nuclear weapons states with atomic energy for peaceful purposes, such as nuclear medicine. Is Iran getting ready to charge that the IAEA is abdicating its responsibilities and to further curb cooperation with the agency and its inspectors?