WASHINGTON — Iran could build a nuclear bomb in a year’s time but would need more time to make the weapon usable, US officials told Congress yesterday.
The administration has not concluded that a nuclear Iran is inevitable, Undersecretary of Defense Michele Flournoy said, but considers the nation’s potential to develop and use such weapons a primary national security threat.
Having contended that Iran could amass sufficient highly enriched uranium to build one bomb in roughly a year, General James Cartwright, vice chairman of the Joint Chiefs of Staff, said an additional three to five additional years would be needed before deployment.
Iran is also pursuing an aggressive ballistic missile program and with outside help could produce an intercontinental missile capable of reaching the United States, a top intelligence official told the Senate Armed Services Committee.
The comments were among the Obama administration’s most precise public assessments of Iran’s military abilities and intentions.
A US military attack on Iran is an option to try to stop or slow progress on such a bomb, Flournoy said, but not an attractive one.
President Obama has said he won’t “take any options off the table with respect to Iran,’’ Flournoy said. “Now that means to me that military options remain on the table.’’
Iran can temporarily restrict access to the strategic Strait of Hormuz and “threaten US forces in the region and our regional allies with missiles,’’ added Lieutenant General Ronald Burgess, director of the Defense Intelligence Agency.
The officials would not publicly address whether the United States has changed its four-year-old assessment that Iran isn’t actively seeking a bomb. The government is preparing a new classified assessment. It is likely to conclude that Iran is at least three years from having a fully usable bomb.
US officials, speaking on the condition of anonymity because the findings are classified, said Iran’s recent testing and other development operations suggest a more active pursuit of weapons technology, but the officials cautioned that the intelligence assessment may not conclude that Iran is now on a full drive to acquire a bomb.
The United States is leading a drive to apply new economic sanctions on Iran. The idea is that if an economically strapped Iran concludes it will lose friends, trading partners, and international standing by developing nuclear weapons, it might choose not to do so.
State Department Undersecretary William Burns predicted that a resolution will emerge from the United Nations Security Council within weeks and that China, formerly a holdout, will go along.
Iran’s nuclear chief said yesterday that his country has produced 5 kilograms of 20 percent enriched uranium for a medical research reactor, a move in open defiance of the UN’s demand that it halt the program. That level of enrichment would be below the 80 percent or 90 percent needed to build a weapon.
Western negotiators thought they had a deal with Tehran last fall that called for enrichment of the uranium in Russia and its return for use in the Iranian reactor. Iranian leaders backed out of the deal.
Yesterday, Ahmet Davutoglu, Turkey’s minister of foreign affairs, said Iran might be willing to restart talks on swapping the uranium. “There is a positive development and a change in approach,’’ he said.
One European diplomat was skeptical that the moves by Iran were anything more than an effort to delay a new round of sanctions.
Farah Stockman of the Globe staff contributed to this report.