Iran has increased the number of operating centrifuges at its uranium enrichment plant to 4,000, a top official said Friday, pushing ahead with the nuclear program despite threats of new UN sanctions. The number was up from the 3,000 centrifuges that Iran announced in November that it was operating at its plant in the central city of Natanz. Still, it is well below the 6,000 it said last year it would operate by summer 2008, suggesting the program may be behind schedule. Deputy Foreign Minister Ali Reza Sheikh Attar, who visited Natanz last week, said Friday that Iran was preparing to install even more centrifuges, though he did not offer a timeframe. "Right now, nearly 4,000 centrifuges are operating at Natanz," Attar told the state news agency IRNA. "Currently, 3,000 other centrifuges are being installed."