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The US and Russian presidents meet at an Asia-Pacific summit here Sunday with Barack Obama eager to secure Moscow's backing to break the stand-off over Iran's nuclear ambitions.
Obama will want to hear from Russian President Dmitry Medvedev whether the Kremlin detects any signs of Tehran moderating its defiance and whether Russia will back tough sanctions if diplomacy fails.
But in their fourth meeting since April, Medvedev will also be seeking assurances from Obama that Russian concerns will be addressed in a new nuclear weapons reduction treaty that both sides are racing to agree by December.
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 an unmatched power of leverage in Tehran.
Russian Deputy Foreign Minister Sergei Ryabkov made a surprise visit to Tehran ahead of the Asia-Pacific Economic Cooperation (APEC) forum in Singapore, declaring there was still hope for resolving the stand-off.
"I do believe that there is room for compromise and improvement if the political will is there, and this is consistently the view of Moscow on this issue," he told Iranian television.
An Obama administration official said that aside from Iran and the nuclear arms treaty, the two leaders would also dwell on North Korea's own nuclear quest.
"We will continue the very constructive and ongoing series of meetings that the president has had with President Medvedev," said Ben Rhodes, Obama's deputy national security advisor for strategic communications.
Obama and Medvedev are scheduled to meet on the margins of APEC on Sunday afternoon, shortly before the US president leaves Singapore for a state visit to China.
Obama is also likely to bring up Iran and North Korea with Chinese President Hu Jintao in the coming week in Beijing.
China and Russia, both permanent UN Security Council members, have been sceptical about ratcheting up sanctions on Iran.
But there is mounting impatience in the West, with world powers still waiting for Tehran to respond to an offer brokered by the UN nuclear watchdog which would see states including Russia help Iran enrich uranium.
The West suspects Tehran is trying to develop a nuclear weapon under cover of its civilian nuclear energy programme.
Like Iran, Russia has said there is no evidence to support these accusations. But it has also urged Tehran to show maximum transparency and cooperate with the international community.
Medvedev has given carefully worded statements hinting that tougher sanctions cannot be ruled out.
"I do not want that all this ends up with the adopting of international sanctions because sanctions, as a rule, lead in a complex and dangerous direction," he told the German magazine Der Spiegel earlier this month.
"But if there is no movement forward then no one is going to exclude such a scenario."
Medvedev is meanwhile set to press Obama about a new nuclear weapons reduction agreement to replace the 1991 Strategic Arms Reduction Treaty (START).
With the old treaty due to run out on December 5, Russian officials have insisted that the new agreement must establish a link between missile defence systems and strategic arms.
Obama's move to scrap missile shield plans in eastern Europe appeared to have lent fresh impetus to the negotiations.
But there remain reported differences on limits to the number of "carriers" that can deliver warheads such as ground-based ballistic missiles, submarines and heavy bombers.
Yet with diplomats working flat out at talks in Geneva to finalise a document, both sides say they are determined to clinch agreement by December.
"We are working for the new agreement to be ready within the deadline set by the two presidents," said Sergei Prikhodko, top foreign policy aide to Medvedev, according to the ITAR-TASS news agency.
From the Obama administration, Rhodes said: "We would like to complete that agreement by the end of the year."
