President Obama’s biggest foreign policy gamble appeared to pay off last night as Russia opened the door to punishing new sanctions on Iran to halt its nuclear programme.

Emerging from his first meeting with Mr Obama since the Eastern Europe missile shield was scrapped, President Medvedev of Russia conceded that “in some cases, sanctions are inevitable”.

Mr Obama went into the meeting, on the sidelines of the United Nations General Assembly, ready to press Russia to support sanctions if Iran refused to address concerns about its nuclear activities. He emerged saying that Mr Medvedev had agreed that “serious additional sanctions” must be considered if diplomatic efforts fail.

That stance was reiterated later by David Miliband, the Foreign Secretary, who emerged from a meeting of foreign ministers from the E3 + 3 countries – Britain, France, Germany, the US, Russia and China — to declare a united front on a policy of diplomacy and sanctions. Mr Miliband called for a “serious response” from Iran at talks scheduled for October 1 if it wished to avoid sanctions.

Mr Medvedev’s – admittedly lukewarm – support for sanctions is seen as payback for Washington’s decision to move its defensive missile shield from Poland and the Czech Republic to the Mediterranean. China was the only remaining Security Council power opposing sanctions on Iran, but the statement from the E3 + 3 suggested that it too was ready to consider them.

In his address last night, Mahmoud Ahmadinejad, the Iranian President, failed to mention the nuclear issue but sparked a walkout by a handful of Western countries, including Britain, when he slammed Israel as racist and denounced Jewish influence in the world.

There had been speculation that he would use even harsher rhetoric to deflect the focus from his disputed re-election and mounting allegations of human rights abuses.

Diplomats said that the date of October 1 had been set to give world leaders time for bargaining during the Assembly. Western officials were pressing Arab leaders to get behind the effort. They met delegates from the Gulf Co-operation Council to discuss how to target Iran’s energy sectors.

In 2006 the UN Security Council voted to ban the supply of nuclear- related technology and materials to Iran and to freeze the assets of key individuals and companies related to the enrichment programme. Two years later, the EU voted to impose financial sanctions. New sanctions under consideration range from a ban on foreign investment and the encouragement of capital flight to put pressure on Iran’s currency to a ban on the export of equipment or technology for oil and gas exploration. A ban on refined petroleum exports is being considered.

Arab states have expressed alarm at Iran’s nuclear capabilities but do not want to ally themselves with Israel. Saudi Arabia is trying to lure Russia into cancelling its S-300 anti-missile system deal with Iran in return for buying $2 billion of its arms. Inducements such as oil deals and jobs for migrant workers could help to persuade China that their interests lie west of Tehran.

In his address to the assembly, President Sarkozy of France warned Iran that it would be making a “tragic mistake” if it underestimated worldwide concern over its nuclear ambitions. He said later that December was the deadline for Iran to prove that the diplomatic track was worth pursuing.

Mr Ahmadinejad indicated in an interview, however, that he planned to stretch out the negotiations. He insisted that he only wanted uranium enriched to 20 per cent — well below the 90 per cent required for weapons grade — for a small research reactor. Until now, Iran has produced only low-enriched uranium. If his request is refused, it could use this as a pretext to produce highly-enriched material.

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