"Iran claims it is not trying to acquire nuclear weapons," Russian news agencies quoted Nikolai Patrushev, secretary of the security council, as saying.

"But actions such as starting to enrich low-enriched uranium up to 20 percent raise doubts in other countries and these doubts are fairly well-grounded," Patrushev was quoted as saying.

His comments were an unusual expression of concern from Moscow, which has for years said there is no evidence that Iran is pursuing anything but a civilian nuclear energy programme as it claims.

The West has long said it fears Iran is using its nuclear energy programme as a cover for development of nuclear weapons, a charge Tehran denies.

Iran declared Tuesday it has started the process of producing 20 percent enriched uranium, defying world powers who have warned of new sanctions unless the Islamic republic halts its sensitive nuclear drive.

Russia has long-standing ties with Tehran and is helping the Islamic republic construct its first nuclear power station.

But though Moscow supports Iran's right to use nuclear technology to generate power, the Kremlin has also signalled that it may support tough new sanctions if Tehran fails to comply with international demands on its nuclear programme.

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