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 intent.

"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.

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.

Patrushev's comments were an unusual expression of suspicion from Moscow, which has for years said there was no evidence that Iran was pursuing anything other than a civilian nuclear energy programme as it claimed.

Hours after Patrushev spoke, the Russian foreign ministry said the decision to ramp up uranium enrichment "heightens doubts on the sincerity" of the Islamic republic's nuclear intentions.

Russia was "disappointed" that Tehran did not resolve the dispute through diplomacy and rapped Iran for flouting UN Security Council resolutions and the International Atomic Energy Agency's decisions.

The statements came after a string of recent remarks from influential Russian politicians that analysts said 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.

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

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.

Foreign Minister Sergei Lavrov warned last month that Iran could not keep the world waiting forever, while Deputy Prime Minister Sergei Ivanov on Saturday voiced impatience at Iran's failure to heed diplomatic efforts.

"We think we should continue this track, but we can't run this track endlessly," Ivanov said, speaking on the sidelines of an international security conference in Munich, Germany.

Patrushev indicated Tuesday that 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.

Fyodor Lukyanov, editor of the foreign policy journal Russia in Global Affairs, said Moscow's abrupt change in tone on the Iran nuclear issue proved Russia was "irritated" by Tehran's latest moves.

"It certainly looks like Russia is taking a tougher stance," Lukyanov told AFP.

"Russia will now be more definite in its statements" on Iran, he predicted.

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