By Jonathan Marcus
BBC Diplomatic Correspondent
Russia and the US reached an outline arms agreement in Moscow
A senior US arms control expert has stressed the informal link between a new strategic arms agreement with Russia and putting pressure on Iran.
Speaking in London in the wake of a US-Russia summit, Gary Samore was cautious about the chances of encouraging Iran to halt uranium enrichment.
But he said a deal on the arms treaty could make Russia much more willing to help toughen sanctions against Tehran.
Mr Samore is the Obama administration's top arms control official.
President Obama's "nuclear man", he is the special assistant to the president for a range of issues including arms control and proliferation.
Speaking at the International Institute for Strategic Studies in London, he said Iran was currently very much focused on its own domestic situation, and until things were more settled there would be no serious diplomacy.
Nonetheless he in effect warned Tehran that if the Obama administration's effort to engage Iran did not produce any results, it would be inevitable that Tehran would face much stronger action from the UN Security Council in New York.
The improving relationship between Russia and the US, he went on, made this a more credible threat.
In the wake of the Moscow summit the informal linkage between a new strategic arms agreement and Russia's approach to Iran is fast becoming clearer.
Mr Samore agreed that Iran was a much more important priority for Washington than for Moscow.
But he argued that Moscow badly needed the new strategic arms agreement, since much of its existing arsenal was ageing and would soon have to be withdrawn from service.
So he believed a deal could be done that would see a new treaty by the end of this year and a much greater willingness by Russia to back tougher sanctions against Tehran.