The top U.S. official working on an international treaty to reduce cigarette smoking worldwide has resigned, at a time when the United States is embroiled in contentious negotiations with more than 150 countries on how to counter the rising global use of tobacco. Bush administration officials said yesterday that the negotiator, Thomas Novotny, has stepped down for personal reasons. But three people who have spoken with him in recent weeks said he had privately expressed distress over the administration’s decision to soften the U.S. positions on key issues, including restrictions on secondhand smoke and the advertising and marketing of cigarettes.