The United States on Wednesday demanded Iran provide detailed answers about its nuclear program after a critical report from the UN atomic watchdog, as Tehran warned it might suspend cooperation with the agency."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," US Secretary of State Condoleezza Rice said Wednesday.Traveling to a conference on Iraq outside Stockholm, Rice referred to the "serious concern" voiced by the International Atomic Energy Agency that Iran was hiding information about alleged studies into making nuclear warheads and defying United Nations demands to suspend uranium enrichment.