AFP - US President Barack Obama on Sunday won the strongest backing yet from Russia on the Iranian nuclear crisis as he warned that Tehran was "running out of time".

Russian President Dmitry Medvedev said after talks with Obama that neither he nor the US leader were satisfied with the pace of progress, as Tehran drags its feet over its response to a UN-sponsored nuclear enrichment deal.

Obama described as "fair" the proposal, which would see states including Russia help the Islamic republic enrich uranium.

"Unfortunately, so far at least, Iran has been unable to say yes," he said. "We now are running out of time with respect to that approach."

Russia has the strongest ties with Iran of any major power, and its capacity to provide technical help for the Iranian nuclear drive is seen by some analysts as giving it unmatched leverage in Tehran.

Medvedev said that Moscow was "not completely happy about the pace" of efforts aimed at defusing the stand-off with Iran.

"Our goal is clear, it is transparent," he said. "We are prepared to work further" to ensure Iran's nuclear programme is only for peaceful purposes, he said.

"In case we fail, the other options remain on the table, in order to move the process in a different direction," he added in reference to new sanctions against Tehran.

"As reasonable politicans we understand that any process should have a final point. The process of talks exists not for the pleasure of talking but for achieving practical goals."

Obama, who badly needs Russia's help to break the stand-off over Iran's nuclear ambitions, exchanged a warm handshake with Medvedev at the conclusion of their talks on the margins of an Asia-Pacific summit in Singapore.

"I have found, as always, President Medvedev frank, constructive and thoughtful," he said.

"The reset button has worked," he added, after years of mutual mistrust.