The United States has reacted with scepticism to Iran's latest effort to prove its nuclear program is for peaceful purposes only. Yesterday, Iranian President Mohammad Khatami took a group of reporters to an underground nuclear facility at Natanz to disprove reports that limited work was going on there despite an agreement to uspend operations. Natanz, some 250 kilometers south of Tehran, was a closely-guarded secret until late 2002 when its existence was revealed by an Iranian exile group. Associated Press reports the journalists were n-o-t shown the plant's centrifuges- key equipment for enriching radioactive material which can be used for either fuel or weapons. U.S. State Department spokesman Adam Ereli dismissed the tour as a "staged media event." Ereli said the Iranians should be answering questions at the International Atomic Energy Agency about what they are doing at Natanz and elsewhere.