GENEVA, Switzerland _ World powers Saturday gave Iran two weeks to accept a freeze on expanding its uranium enrichment work as a step toward full-scale negotiations on its nuclear program's future, or face new economic sanctions and isolation. The powers told Iran that there would be no further talks on their offer to withhold new sanctions for six weeks in return for Iran not adding new enrichment machines called centrifuges to its plant at Natanz for a similar period. "The Iranians know very well what will continue to happen (with further sanctions)," European Union foreign policy chief Javier Solana said after talks with Iranian negotiator Saeed Jalili. Jalili failed to give a response to the "freeze for a freeze" offer the powers made a month ago to encourage Iran to heed U.N. demands to suspend its enrichment effort and open negotiations on the program it hid from international inspectors for 18 years. The U.S., Israel and others believe Iran is secretly developing nuclear weapons. Iran insists it is legally enriching uranium for fuel for power plants. The inconclusive outcome dimmed hopes of movement in the six-year dispute over Iran's nuclear program raised by the presence at the talks of Undersecretary of State William Burns, the third highest U.S. diplomat. It was the first time a U.S. official has attended face-to-face discussions with Iran on its nuclear program, reversing a Bush administration policy not to attend such a session until Iran heeded U.N. demands to suspend its enrichment of uranium. The session in Geneva's ornate city hall was also attended by senior diplomats from Britain, France, Germany, China and Russia.