"We do not believe that Iran has made the strategic decision to abandon its efforts to develop a nuclear weapons capability," said State Department spokesman Richard Boucher.
Boucher added, "We do not believe that Iran has been fully transparent in its October declaration to the IAEA," the International Atomic Energy Agency, on its nuclear activities.
"Iran has continued to hide information from the IAEA related to its ongoing efforts to develop a nuclear weapons capability," Boucher charged, also citing its alleged "failure to suspend all enrichment related and reprocessing activities."
Boucher did not say whether Washington planned to take the case to the UN Security Council to seek possible sanctions.
Referring to a pending report from the IAEA chief Mohamed ElBaradei on the subject and a meeting of the agency's directors, Boucher said "we are looking forward to the next report from the director general of the AIEA. That report is expected soon."
And "the board will meet again on March 8 to 10 and at that time the board can judge whether at that moment Iran is meeting its pledges."
Iranian Foreign Minister Kamal Kharazi said in Rome that his country had no plans to develop nuclear weapons and reaffirmed its willingness to cooperate with the IAEA.
Later Friday, the Republican-controlled US Senate unanimously passed a resolution of concern over Iran's failure to meet its IAEA obligations.
The resolution expressed "the deep concern of Congress regarding the failure of the Islamic Republic of Iran to adhere to its obligations under a safeguards agreement with the International Atomic Energy Agency and the engagement by Iran in activities that appear to be designed to develop nuclear weapons."