Iran has rebuffed a bid from the UN nuclear monitoring agency to beef up its monitoring ability at an important atomic site as it tries to keep track of the country's rapidly growing uranium enrichment capabilities, diplomats said Thursday. The diplomats said the Islamic Republic in recent weeks turned down a request from the International Atomic Energy Agency to place one or more additional surveillance cameras at the Natanz enrichment site. In addition, they said, the agency was concerned Iran would use its recent denial of access to Natanz to agency inspectors seeking a surprise visit as a precedent, further hampering the UN agency's need to increase its oversight. IAEA spokesman Marc Vidricaire said the agency would have no comment. The three diplomats - all from IAEA member nations - said it was possible Iran would reconsider, emphasizing that talks continued between the agency and Iranian officials. They demanded anonymity because their information was confidential. Still, Iran's reluctance to allow the agency to upgrade its monitoring is troubling at a time of rapid expansion of the number of uranium enriching machines and their ability to produce material that could be upgraded into weapons-grade uranium. Since Iran's clandestine enrichment efforts were revealed more than six years, ago, the country has steadily expanded activities at its cavernous underground facility at Natanz, a city about 300 miles (500 kilometers) south of Teheran. An IAEA report circulated last week 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. Iran says it is interested in producing only low-enriched uranium for fuel use, not highly enriched material for the fissile core of nuclear weapons, and the international nuclear agency has detected no effort at Natanz to contravene its assertion. Still, if Iran decided to risk an international crisis by reconfiguring its centrifuge setup, it would have the ability to process its low-enriched material into weapons-grade uranium. Most experts estimate that the over 1,000 kilograms - 2,200 pounds - of low-enriched uranium Iran had accumulated by February was enough to produce enough weapons-grade material through further enrichment for one nuclear weapon. And as it expands its operations at Natanz, its potential capacity to produce highly enriched uranium is also growing. Last week, the Washington-based Institute for Science and International Security estimated that, with the nearly 5,000 centrifuges now fully operating, Iran could accumulate enough material to produce weapons-grade uranium for two warheads by February 2010 - or sooner, if it brought the more than 2,000 additional machines on line immediately. Iran steadfastly refuses to stop enriching despite the imposition of three rounds of economic, trade and financial sanctions by the UN Security Council. Even if hardline President Mahmoud Ahmadinejad loses Iranian presidential elections starting Friday, expectations are low that the country will change its nuclear stance. Last week's IAEA report - prepared for a meeting starting Monday of the agency's 35-nation board of governors - touched on concerns about being able to keep track of Natanz operations. It said the agency had informed Iran that, due to the growth in enrichment capacity and output, it was seeking "improvements to the containment and surveillance measures" it now had at hand. And a senior UN official said expansion at Natanz "makes it increasingly difficult to do the surveillance" needed to ensure none of the material produced is being diverted. To do its work at Natanz, the agency relies in part on monitoring by cameras and on inspections meant to give the Iranians a minimum of time between the announcement of the visit and the arrival of the inspectors - methods the agency would like to expand, said the diplomats. They said that Iran's refusal to allow any additional cameras was a setback, along with its recent denial of an IAEA spot inspection. Last week's report referred to that denial, noting that - while Teheran allowed 25 unannounced inspections at Natanz since 2007, it blocked the request last month arguing the facility was off-limits because of a security drill. "The agency is pushing to make sure Iran does not use this incident as a precedent" to deny further access to Natanz, said one of the diplomats, who is regularly briefed on the IAEA's Iran file.