The International Atomic Energy Agency report issued earlier this week in Vienna said Tehran should provide more information on its missile-related activities and was holding back information on high-explosives testing relating to its nuclear programme.
"I think right now the Iranians have a lot of explaining to do about the IAEA report, which essentially sees them as not cooperating on some very important dark questions that the international community has about their programs," Rice told reporters on her plane on the way to an international conference on Iraq in Stockholm.
"The major question on my mind today is: how are the Iranians are going to answer the quite serious charges of non-cooperation," Rice said.
The IAEA report said Tehran had 3,500 uranium enrichment centrifuges working at its Natanz nuclear facility, slightly more than earlier this year, and a few more advanced centrifuges were also being tested.
The nuclear watchdog has been pressing Tehran for answers to Western intelligence allegations that Iran has covertly studied how to design atomic bombs. Iran has dismissed the intelligence as baseless, forged or irrelevant.
Rice said the United States would continue along two tracks of action — involving sanctions as well as incentives — aimed at getting Iran to give up uranium enrichment.
She noted that the European Union's foreign policy chief, Javier Solana, hoped soon to present Iran with a new package of incentives from the five permanent members of the U.N. Security Council plus Germany.
Rice ignored a question as to whether she planned to meet Iranian Foreign Minister Manouchehr Mottaki in Stockholm. U.S. officials have said there are no plans for her to do so.
Iran has been subjected to three rounds of U.N. Security Council sanctions over its nuclear program.
Washington has imposed sanctions on more than 20 Iranian companies, banks and individuals as well as the defense ministry, hoping to increase pressure on Tehran to stop uranium enrichment and curb what the United States views as terrorist activities. Tehran denies the charges and says its nuclear program is aimed solely at producing electricity.
Rice said earlier this week that Iran should expect more sanctions but gave no details.
(Reporting by Susan Cornwell, editing by Tim Pearce)
