Those activities could include a four-year program to validate the design of a neutron initiator, a device that releases a burst of neutrons to “kick start” a nuclear chain reaction in a bomb core, the report said.

The report could create a new policy headache for President Barack Obama. He has pursued a two-track approach toward Iran of sanctions and other measures _ the United States and Israel are believed to have developed a computer virus that slowed Iran’s uranium enrichment effort _ and holding open an offer to negotiate a settlement with Tehran.

The sanctions, both international and unilateral, have seriously hurt Iran. But they have failed to force President Mahmoud Ahmadinejad’s regime into bowing to repeated U.N. demands to halt the program until Iran convinces the IAEA that it is for peaceful purposes.

With the campaign for the 2012 election intensifying, Obama could face charges by conservatives that his approach is too soft.

But while the administration maintains the option of military action against Iranian nuclear facilities remains open, the European allies and other powers firmly oppose the use of force, which could trigger a violent response and send petroleum prices soaring as the world struggles to avert an new economic crisis.

The Obama administration withheld formal comment while it studied the report. Senior administration officials conceded that they are unlikely to win the support of Russia and China _ veto-wielding permanent members of the U.N. Security Council _ for a fifth round of international sanctions.

But a senior administration official, who requested anonymity in order to discuss the sensitive issue, said that the United States is consulting with European allies and other countries on intensifying bilateral sanctions that have helped ground “the Iranian economy ... to a halt for the first time in decades.”

The report’s release was preceded by intense Israeli media speculation of impending Israeli airstrikes on Iran’s nuclear facilities, which the Israeli government views as an existential threat to the Jewish state.

Just prior to the report’s release, Israeli Defense Minister Ehud Barak held out the possibility of military action, saying that Iran is unlikely to bend to additional sanctions.

“We continue to recommend to our friends in the world and to ourselves not to take any option off the table,” he said, using a euphemism for military strikes.

Russia and China, which reluctantly supported four rounds of U.N. sanctions against Tehran for defying demands to suspend its uranium enrichment program, sought to delay the report’s publication, portraying it as a hindrance to international efforts to negotiate a diplomatic resolution with the Islamic republic.

Moscow called on Tuesday for time to study the new report.

"The analysis should be carried out in a calm atmosphere because it is important to figure out whether there really are new, and indeed trustworthy, facts that confirm the suspicions that there are military components in the Iranian nuclear program, or whether we're talking about the intentional and counterproductive exacerbation of emotions," the Russian Foreign Ministry said.