Speaking to journalists in Moscow, the powerful head of Russia's national security council, Nikolai Patrushev, said Tehran's decision to enrich uranium to higher levels itself gave rise to "well-grounded" doubts about its plans.

"Iran claims it is not trying to acquire nuclear weapons," Patrushev was quoted as saying by Russia's state-controlled news agencies.

"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 was no evidence that Iran was pursuing anything but a civilian nuclear energy programme as it claimed.

They were also the latest in a string of remarks from influential Russian politicians in recent days that analysts said together amounted to a clear effort by Moscow to telegraph a possible change of tack on Iran nuclear policy.

"There is no doubt that Russia is toughening its position on Iran," Alexander Konovalov, an analyst at the Institute for Strategic Assessments told AFP, calling the comments a "clear indication that Russia is displeased.

"I have no doubt that when the question comes up Russia will support a new round of sanctions," against Iran, Konovalov said.

Iran declared Tuesday it had 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.

The United States and France have warned they will push for "strong" new UN sanctions against Tehran.

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.

The International Atomic Energy Agency said Tuesday that a team of its inspectors was in place in Iran to monitor Tehran's plans to start enriching uranium.

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 nuclear technology to generate power, the Kremlin has also signalled that it may back new sanctions if Tehran refuses to comply with international demands on its nuclear programme.

Patrushev indicated the Kremlin's patience in trying to seek dialogue with Iran was wearing out.

"Political and diplomatic methods are important for regulating, but everything has its limit," Patrushev was quoted as saying Tuesday.

He called for Iran to take part in "open cooperation with the International Atomic Energy Agency."

Foreign Minister Sergei Lavrov warned last month that Iran could not keep the world waiting forever.

"If we do not see a constructive reply from Iran's side, we will need to talk about that at the UN Security Council," Lavrov said Friday in Berlin.

An influential Russian lawmaker, Konstantin Kosachyov, chairman of the Duma foreign affairs committee, on Tuesday called for tougher sanctions on Iran.

"Russia should put more pressure on Iran in consolidation with its partners in the six," Kosachyov told the Echo of Moscow radio station, referring to the six world powers attempting to convince Iran to abandon sensitive nuclear work.

Kosachyov called for energy sanctions to be imposed on Iran, saying that "one of the most painful options would be limits on supplies of energy to Iran, particularly petrol."

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