Russian President Dmitry Medvedev has eased the diplomatic pressure off the Kremlin by adamantly telling Iran that it must "prove" to the world that its nuclear drive was entirely peaceful.

Medvedev has recently taken on a series of cautious but consistent steps to reverse Moscow's unwavering backing for its traditional ally -- a policy that posed one of the main stumbling blocks to its relations with Washington.

He met Iranian President Mahmoud Ahmadinejad in the Azerbaijani capital Baku on Thursday for a closed-door meeting that the Kremlin admitted was heated but focused on resolving the two sides' ever-growing list of disputes.

But Medvedev made clear at a wide-ranging press conference in Lisbon on Saturday that his patience with the Islamic nations was ending and that he was now expecting "good will" from Ahmadinejad.

"Iran has the right to the peaceful atom. But it must prove to everyone that its nuclear industry development programme is peaceful and that it is ready to cooperate with international organisations," the Russian leader said.

"We are not indifferent ... as to who is an active member of the nuclear club (of nations) and who is only trying to make it into that club -- who is trying to legitimize themselves by saying that they have nuclear weapons and who is hiding them."

Medvedev said that he spelled all this out to Ahmadinejad during their exchange.

"We discussed the state of Russia-Iran relations -- relations that for quite obvious reasons have not been the easiest as of late," said Medvedev.

The comments came at the end of a high-profile Russia-NATO summit in Lisbon that the Alliance used to officially proclaim that it "poses no threat to Russia".

But it is Russia's engagement with Iran -- most notably through its construction of the country's first nuclear power plant and steady supply of sensitive arms -- that has led to the greatest number of diplomatic questions.

And Medvedev made a point of stressing that Russia and NATO were now combining their efforts on Iran.

"We will also be joining NATO countries to carefully examine how the various (nuclear) programmes develop," Medvedev said.

He first altered his message in September 2009 following Iran's disclosure that it had secretly built a second uranium enrichment plant.

Yet Medvedev at the time spoke of the need "to create comfortable conditions and a system of stimuli" that assure future Iranian cooperation.

Analysts detected a far more strident note in Medvedev's latest message to Iran.

"Russia is clearly irritated that Iran is not cooperating with the international community even though time is running out," said Carnegie Moscow Center Deputy Director Dmitry Trenin.

He said that US President Barack Obama's decision to welcome potential dialogue with Tehran has made it that much harder for Moscow to justify Tehran's decision to stall on negotiations.

"Iran no longer has the excuse of United States not willing to talk. And this has made Russia realize that there must be a real reason why Iran is dragging things out."

Trenin added that both Medvedev and Prime Minister Vladimir Putin -- who some analysts see as the real leader of Russia and who has held a much softer line on Iran -- were now in complete agreement on the issue.

"Russia -- in the face of Medvedev but also in the face of Putin -- believes that the time is right to put up a common diplomatic front on Iran."

Medvedev's message was picked up by a Russian state media that echoes the latest shifts in Kremlin tone.

"Iran must prove that it is willing to cooperate with the IAEA," the Voice of Russia state radio station said in reference to the United Nation's nuclear inspection body.

But some analysts said that Medvedev may yet regret his decision to drop support for a nation that provides Russia with vast opportunities to reap huge profits from various military and energy deals.

"Russia has started using pedantic and I would almost say rude language in its latest dealings with Iran," said Rajab Safarov of the Contemporary Iranian Studies Centre in Moscow.

"This is a big mistake," said Safarov. "We are permanent neighbours and this is where Russia's real interests lie."