Burns said the United States fully expects the UN's nuclear watchdog agency, the International Atomic Energy Agency, to refer Iran to the UN Security Council if such work does not cease.
Burns described Iran as acting in "defiance" of the international community by refusing to halt the nuclear-fuel work. He did not rule out, however, that Iran could resume negotiations with Britain, France, and Germany, saying Washington still believes there is life in the diplomatic process.
Iran denies having any covert program to develop a nuclear weapon.
For RFE/RL's complete coverage of the controversy surrounding Iran's nuclear program, see "Iran's Nuclear Program."