Russian president Dmitry Medvedev has called on Iran to explain the "military components" of its nuclear programme.
Russia has fostered lucrative trade ties with the Islamic Republic over the past two decades but the Kremlin has struck a distinctly worried tone about the potential threat from a nuclear-armed Iran.
"I would like to say that Iran is our rather active trading partner and has been tested by time, but that does not mean we are indifferent to the way Iran is developing its nuclear programme and we are not indifferent to how the military components of the corresponding programme look," Mr Medvedev said.
"In this respect, we are waiting for the appropriate explanations from Iran," he said at a news conference with German chancellor Angela Merkel in the Urals city of Yekaterinburg.
US president Barack Obama has sought to win Kremlin support for a tougher line against Tehran, and in June Moscow supported further United Nations Security Council sanctions against Iran.
Western powers suspect Iran is trying to build a nuclear bomb, something it has repeatedly denied. Russian diplomats were insisting in public as late as last year that they had seen no evidence that Iran was seeking to make nuclear weapons.
But in a speech delivered on Monday, Mr Medvedev for the first time warned that Iran was moving closer to getting such weapons, the frankest assessment from a Kremlin leader in at least a decade about the alleged threat from Iran's nuclear programme.
"Russia is being even more forthright with its concerns about the Iranian nuclear programme, although I don't know if there was any particular piece of information that caused this," said one Western diplomat who asked not to be identified.
Since 2005, the United Nations nuclear watchdog has been reviewing Western intelligence reports indicating that Iran has co-ordinated efforts to process uranium, test explosives at high altitude and revamp a ballistic missile to make it suitable for a nuclear warhead.
Iran says its nuclear programme is peaceful and aimed solely at generating electricity, and that Western powers are using false intelligence reports to manipulate the UN watchdog.
Mr Medvedev, who was sworn in as Russian president in May 2008, said both open sources and reports from secret services showed that Iran was developing its military nuclear programme. "Iran must find the courage to start fully fledged cooperation with the international community," he said.
Prime Minister Vladimir Putin, Russia's most powerful politician and the ultimate arbiter on major policy decisions, rarely comments on Iran, though he warned last year against intimidating Tehran.
Diplomats said Moscow has proven adept at sending mixed signals on the issue, pursuing lucrative business contracts but also supporting UN sanctions resolutions against Tehran in return for concessions from the West.
Russia is preparing to start up the reactors at Iran's first nuclear power station, which it aims to open up later this year and Moscow sought yesterday to soothe Tehran's worries about sanctions by mapping out an ambitious plan for energy deals.
Reuters