"It does not seem wise to put increasing trust in Iran as a source and transit country of natural gas" when it is under sanctions over its controversial nuclear programme, the agency quoted Kathryn Schalow, the spokeswoman for the US embassy here as saying.

The memorandum of understanding was signed away from public eye last week in Ankara after talks between Turkey's Energy Minister Hilmi Guler and Iranian Oil Minister Kazem Vaziri Hamaneh.

The deal is seen as boosting the prospects of a planned 3,300-kilometre (2,000-mile) pipeline project, known as Nabucco, to carry gas from the Middle East and Central Asia to the European Union via Turkey and the Balkans, bypassing Russia.