A top US diplomat heading to Geneva has no plans to meet separately with Iran's chief nuclear negotiator, but the mere presence of the Bush administration official at talks between the Iranian negotiator and representatives of other world powers will be a sharp break with past administration policy. William Burns, America's third highest-ranking diplomat, will attend talks with the Iranian envoy, Saeed Jalili, in Switzerland on Saturday. The talks are aimed at persuading Iran to halt activities that could lead to the development of atomic weapons. It will be the first time such a high-ranking US official has attended such talks. Official contacts between Iran and the United States are extremely rare and although Washington is part of a six-nation effort to get Iran to stop enriching and reprocessing uranium, the administration has shunned contacts with Tehran on the matter. White House press secretary Dana Perino referred to the weekend meeting as a "one-time US participation."