Iran's belated revelation of a second uranium enrichment site has raised concerns about possible further secret nuclear sites in the country.
The International Atomic Energy Agency (IAEA) raised its fears in a report obtained by several news agencies on Monday.
The UN nuclear watchdog's report said Iran had claimed that work on the second bunkered site near Qom had started in 2007.
However, the IAEA says it has evidence the project began in 2002, was paused in 2004 and resumed in 2006.
"The agency has indicated [to Iran] that its declaration of the new facility reduces the level of confidence in the absence of other nuclear facilities under construction and gives rise to questions about whether there were any other nuclear facilities not declared to the agency," the report said.
The report is the first official document since IAEA experts inspected the Qom site last month.
The report said that the plant "corresponds to the design information provided by Iran" but that "Iran's explanation about the purpose of the facility and the chronology of its design and construction requires further clarification".
Iran revealed to the IAEA in September that it had been building a second uranium enrichment plant inside a mountain near Qom.
The inspectors said the plant could go live as early as 2011.
The IAEA inspectors also found that Iran had recently reduced the number of centrifuges enriching uranium at its main Natanz site by 650 to 3,936, while slightly raising the total number of machines installed to 8,692.
Western diplomats and analysts said the slowdown was probably caused by technical glitches. [adm/aljazeera]
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