Iran on Wednesday heralded what it called a pair of significant advances in its controversial nuclear research efforts, but Western observers generally downplayed the developments as more hype than substance.
The official focus on nuclear "achievements" was aimed at showing domestic and international audiences that Iran was capable of moving ahead on the nuclear front despite international sanctions based on allegations that it is seeking weapons capability.
Tehran contends that it is pursuing nuclear research only for peaceful uses, such as generation of energy and treatment of cancer patients. But concern that Iran may be developing a weapons program has triggered broad speculation that Israel or the United States may bomb Iranian nuclear facilities.
Iran has said it would retaliate massively against any such strike.
The prospect of a potential attack on Iran's nuclear facilities has become a major source of debate in U.S. and international policy circles, throwing renewed light on the country's nuclear research program.
Iran has made pronouncements in recent weeks signaling its determination to continue its drive toward what it calls nuclear self-sufficiency.
On Wednesday, President Mahmoud Ahmadinejad, donning a white lab jacket, was seen on state television observing the insertion of what were described as the first domestically produced fuel rods into a Tehran research reactor designed to produce isotopes for the treatment of cancer patients.
A defiant Ahmadinejad said the breakthrough came after Western nations had attempted to keep "science" from the Islamic Republic, denying Iran the fuel needed for the research reactor.
At a ceremony in Tehran marking the reported atomic advances, Ahmadinejad said Iran's nuclear scientists would not be deterred, despite the mysterious assassinations of a number of nuclear experts in recent years.
"Who is afraid of assassination in Iran?" Ahmadinejad said, the state-run news network reported. "Nobody is afraid of assassination."
Still, the president said that security for nuclear scientists would be bolstered.
Separately, various Iranian news outlets reported that a "new generation" of centrifuges was installed at the nuclear enrichment facility at Natanz in central Iran. Additional details were not available, but any sign that Iran's enrichment capability was increasing would probably be a concern to Western observers.
Inspectors for the International Atomic Energy Agency, the U.N.'s nuclear monitor, are scheduled to return to Iran this month. The inspectors are attempting to clarify any potential military dimensions to the nuclear program. Iran hosted the inspectors two weeks ago and said the nation would welcome them again.
In Washington, the Obama administration reacted skeptically to the latest nuclear announcements, contending that the Iranian program was actually behind schedule.
"This is not big news; in fact, it seems to have been hyped," said Victoria Nuland, the chief State Department spokeswoman.
She said the announcement showed that Iran "is clearly feeling the pressure of international and diplomatic isolation, of the increasing pressure on it."
Iran's economy has been in a tailspin, with high inflation and a devalued currency, as the U.S. and other Western nations have tightened sanctions in connection with its nuclear program.
The European Union has announced a cutoff in imports of Iranian oil as of July 1. There were conflicting reports from Iran on Wednesday about whether Tehran had preemptively decided to cut exports to six nations: Italy, Spain, Portugal, Greece, France and the Netherlands.
Times staff writer McDonnell reported from Beirut and special correspondent Mostaghim from Tehran. Times staff writer Paul Richter in Washington contributed to this report.