US President Barack Obama praised Russian President Vladimir Putin for his role in the Iran nuclear deal saying there could now be an “opening” for further detente in the worst crisis in American-Russian relations since the Cold War, the Telegraph reported.

Speaking shortly after a historic agreement to curb Iran's nuclear programme was signed in Vienna, Obama said that there was now an opportunity for a "serious conversation" with Putin about the fate of Bashar Assad, the embattled Syrian president.

He did not go into details, but analysts in Moscow and Washington have previously suggested diplomats may be working on a bargain that would see a managed change of regime in Damascus acceptable to both Moscow and Washington.

Speaking to the New York Times Obama said Putin’s cooperation had “surprised” him, and that he was hopeful of an “opening” for further detente in otherwise fraught relationship between the two powers.

“Russia was a help on this. I’ll be honest with you. I was not sure given the strong differences we are having with Russia right now around Ukraine, whether this would sustain itself. Putin and the Russian government compartmentalized on this in a way that surprised me,” he told the paper.

“We would have not achieved this agreement had it not been for Russia’s willingness to stick with us and the other P5-Plus members in insisting on a strong deal.”