Iran and the US aim in talks that started on Sunday in Switzerland to begin closing in on a deal reducing Teheran's nuclear activities to within strict limits after 18 months of tortuous negotiations.

Time is running short, however, and tempers are fraying in Washington, where critics fear that the mooted accord will not do enough to prevent the Islamic republic from getting nuclear weapons.

US Secretary of State John Kerry, due to meet his Iranian counterpart in Lausanne later, sought to allay such concerns, saying the aim was "not just to get any deal, it is to get the right deal".