Iran will soon announce an atomic breakthrough, the semi-official Mehr news agency reported on Wednesday. "This great scientific achievement is the fruit of a long-term research project ... It will be formally announced by a top official," Mehr quoted an informed source as saying. "The announcement will highlight Iran's mastery of different areas in nuclear science and will reinforce Iran's position as a nuclear country," the report said.Meanwhile, the United States said it was carefully reviewing Iran's reply to an offer meant to end the standoff over Tehran's atomic programmes, but insisted the Islamic republic must freeze sensitive nuclear activities. "Everybody understands that the international community has spoken, that a nuclear programme must end in Iran," said White House spokeswoman Dana Perino, who later clarified she meant that Tehran must halt uranium enrichment.