The US on Wednesday warned Turkey not to strike an energy deal with Iran that undermined diplomatic efforts to halt Tehran’s nuclear programme, on the eve of a visit to Ankara by Mahmoud Ahmadi-Nejad, the Iranian president.

A deal to increase Iranian natural gas supplies to Turkey is expected to form the centrepiece of the visit, giving Mr Ahmadi-Nejad a much-needed diplomatic boost as he attempts to defy pressure to isolate Iran.