Iran's total production of low-enriched uranium has risen by around 15 per cent since May to reach 2.8 tonnes, according to a restricted UN report.
The report, obtained by news agencies on Monday, said that the International Atomic Energy Agency (IAEA), a Vienna-based UN agency, remained concerned about possible activity in the Islamic Republic to develop a nuclear payload for a missile.
"Iran has estimated that, between February 9 2010 and August 20 2010 ... 22 kilogrammes or UF6 enriched up to 20 per cent has been produced" at its pilot fuel enrichment plant in Natanz, the report said.
The White House called the report "troubling" and said that it showed Tehran was still trying to develop a nuclear weapons capability.
"The IAEA's latest report on Iran again demonstrates that Iran is refusing to comply with its international nuclear obligations, and continues its effort to expand its nuclear program and move closer to a nuclear weapons capability," Tommy Vietor, the White House spokesman, said.
Despite four rounds of UN sanctions, Iran is continuing to enrich uranium at its plant in Natanz.
UN inspectors barred
The Islamic republic says it needs the 20 per cent enriched uranium for a research reactor that makes radioisotopes for medical purposes, but the West fears the material is ultimately intended for a nuclear weapon.
Speaking to Al Jazeera from Tel Aviv, Meir Javedanfar, the author of The Nuclear Sphinx of Tehran, said: "Iran has now overcome the biggest hurdle in its enrichment process."
"The distance between 20 per cent, which can be used for both civilian and military [purposes], and 90 per cent enriched uranium, which is for military purposes only, is very small."
"[It] is much smaller than between three and a half per cent to 20 per cent."
The IAEA also voiced concern about what it called Iran's "repeated objection to the designation of experienced inspectors hampers the inspection process and detracts from the agency's ability to implement safeguards in Iran."
"[It] thereby detracts from the agency's capability to implement effective and efficient safeguards in Iran."
The complaint follows Iran's recent decision to strip two experienced inspectors of the right to monitor Tehran's nuclear activities after they reported undeclared nuclear experiments conducted by Tehran.
According to Iran, the reporting by the two experts was inaccurate. But the IAEA said it had "full confidence in the professionalism and impartiality of the inspectors concerned, as it has in all of its inspectors."
The report is being circulated to the IAEA's 35-nation board and to the UN Security Council.