Iran is close to having the potential to build a nuclear weapon, President Dmitry Medvedev said on Monday in the clearest indication yet of Russian alarm over Tehran's atomic drive.

"Iran is nearing the possession of the potential which in principle could be used for the creation of a nuclear weapon," Medvedev said at a meeting with Russian diplomats quoted by Russian news agencies.

Russia, traditionally a diplomatic and economic ally of the Islamic republic, in the past took a milder line against Tehran than Western powers but recently noticeably hardened its position.

Iran has over the past months been announcing steady advances in its nuclear programme, in defiance of international calls for Tehran to freeze its sensitive uranium enrichment operations.

Iranian atomic chief Ali Akbar Salehi on Sunday said Tehran has produced around 20 kilogrammes of 20 percent enriched uranium.

Medvedev said that Iran "is far from behaving in the best way".

Russian last month joined other world powers in approving new sanctions against Tehran. Medvedev repeated his belief that sanctions often do not produce results, but he addat that in Iran's case they could stimulate talks.

"Now what we need is patience and as quickly as possible to renew dialogue with Tehran," Medvedev said.

"This is what we see as the main aim of the UN Security Council resolution. And if diplomacy loses this chance then this will be a collective failure of all the international community," he said.

Western powers accuse Iran of seeking to build a nuclear bomb under the cover of a civilian nuclear energy programme, charges that are fiercely disputed by Tehran.

The United States and Israel have never ruled out the use of military force to end Tehran's defiance, although Russia has always insisted that the standoff should be solved diplomatically.

But Russia had in the past always scoffed at Western suggestions the Iranian nuclear drive was not peaceful and Medvedev's comments were a clear indication that Moscow's trust is dwindling.

Russia's tougher line on Iran has coincided with a warming of its relations with the United States. Washington has repeatedly praised Moscow for its support in the crisis.

The US-Russia espionage scandal that ended last week with the biggest spy exchange since the Cold War initially risked derailing the rapprochement but both sides have insisted it remains on track.

Medvedev did not mention the spy scandal in his speech to diplomats but said Russia and the United States "have no right to pause on the way to smoothing mutual understanding."

"The remnants of the Cold War are receding into the past," said Medvedev. "This rhythm must be the foundation for continuing this work to exploit the potential for mutual understanding in all areas," he added.

Russia's tougher line has already caused an unprecedented slump in its relations with Iran and Iranian President Mahmoud Ahmadinejad has warned Moscow that it risks joining Washington as a historic enemy of Tehran.