VIENNA (AFP) - Iran is continuing to stall on UN investigation into its disputed nuclear programme, refusing to provide access to documentation, individuals or sites which could reveal the true nature of its activities, the UN atomic watchdog said Monday. According to the report, Iran is now operating close to 3,800 uranium gas centrifuges at its enrichment plant in Natanz, 200 more than in May when IAEA published its previous report. Another so-called "cascade" of 164 machines was similarly up and running but was not being fed with uranium gas. Furthermore, the Islamic republic is defying international demands to suspend uranium enrichment, a process that can be used to make the fissile material for an atomic bomb, the International Atomic Energy Agency said. The United States warned Iran could now face possible new sanctions in the wake of the IAEA's findings. The agency complained that it was making little headway in its investigation into allegations that Tehran had, in the past, been involved in studies to make a nuclear warhead. The IAEA "regrettably has not been able to make any substantive progress on the alleged studies and other associated key remaining issues which remain of serious concern," said the restricted report, a copy of which was obtained by AFP. The information collected by IAEA from a wide number of sources was "of such a quality that Iran has to take it seriously" and had to provide a "substantive response," a UN official said. In order for progress to be made, Iran "needs to provide the agency with substantive information to support its statements and provide access to relevant documentation and individuals in this regard," the IAEA said. Furthermore, "contrary to the decisions of the (UN) Security Council," Iran had still not suspended its uranium enrichment activities, the IAEA complained. In fact, Iran had installed additional cascades of uranium-enriching centrifuges, bringing the number up and running to close to 4,000, and was testing more advanced centrifuges as well. So far, Iran's enrichment plant in Natanz has produced a total of 480kg low-enriched uranium or LEU, it said. It would need 1,700kg to convert into high-enriched uranium (HEU) for use in an atom bomb, a UN official said. In Washington, the White House warned that the stalemate in the IAEA's investigation could lead to further sanctions against Iran. "We urge Iran to suspend its uranium enrichment and reprocessing activities or face further implementation of the existing United Nations Security Council sanctions and the possibility of new sanctions," said spokesman Gordon Johndroe. His comments echoed remarks made by IAEA Director General Mohamed ElBaradei in the summary of the report, who urged Iran "to implement all measures required to build confidence in the exclusively peaceful nature of its nuclear programme at the earliest possible date." Meanwhile, Iran's ambassador to the UN atomic watchdog said that his country will continue enriching uranium in defiance of UN Security Council demands, the ISNA news agency reported. "Stating that Iran did not obey the United Nations Security Council resolution asking it to halt uranium enrichment shows this reality - that Iran found no logical and legal reasons for doing so," Ali Asghar Soltanieh said. "Therefore it cannot give up its undeniable right under the International Atomic Energy Agency (IAEA) charter," ISNA quoted him as saying.