A senior US official said on Friday that Teheran was not fully cooperating with the UN's International Atomic Energy Agency. In comments carried by Israel Radio, the top official was quoted as saying that despite Iran's agreeing to let IAEA inspectors examine the heavy water facility at Arak, the Islamic republic was not honoring its commitments. On Thursday, Iran lifted a yearlong ban, allowing UN inspectors to visit the nearly completed Arak nuclear reactor and granting greater monitoring rights at another atomic site, diplomats said. IAEA inspectors visited the Arak heavy water reactor last week, the diplomats told The Associated Press. Separately, they said Teheran agreed last week to IAEA requests to expand its monitoring of the Natanz uranium enrichment site. The diplomats demanded anonymity because their information was confidential. The agency had been seeking additional cameras and inspections of the Natanz site, to keep track of the rapidly expanding enrichment program. Iran's stonewalling had raised agency concerns that its experts might not be able to make sure that some of the enriched material produced at Natanz is not diverted for weapons use. Since its clandestine enrichment efforts were revealed more than six years ago, Iran has steadily increased activities at its cavernous underground facility at Natanz, a city about 500 km. south of Teheran. A June IAEA report said nearly 5,000 centrifuges were now enriching at Natanz - about 1,000 more than at the time of the last agency report, issued in February - with more than 2,000 others ready to start enriching. A new report due in the next week or so is expected to confirm that operations have continued to expand - along with Teheran's potential capacity to produce weapons-grade uranium. Most experts estimate that the more than 1,000 kg. of low-enriched uranium Iran had accumulated by February was already enough to produce enough weapons-grade material through further enrichment for one nuclear weapon. Before lifting the ban on visiting Arak, Teheran had repeatedly refused IAEA inspection requests, despite warnings by the agency that its stance contravened mutual agreements. Western countries have repeatedly called on Iran to stop construction of the reactor, fearing it could be used as a second track toward building a warhead. When finished, say experts, Arak could produce enough plutonium for a nuclear weapon each year.