John Kerry holds second round of nuclear talks with Iranian Foreign Minister in Munich

John Kerry huddled with Iranian Foreign Minister Mohammad Javad Zarif for the second time during their stay in the southern German city, as global powers grapple for a political accord to rein in Iran's atomic ambitions ahead of a March 31 deadline. John Kerry was due to fly back to Washington later in the day.

Few details of the tough negotiations have leaked, but world powers are trying to ensure that Iran's pathway to developing a nuclear weapon are cut off, and in return they will agree to a gradual easing of international sanctions which have crippled Iran's economy.

But skepticism is mounting about whether a deal is possible, despite an interim accord agreed in November 2013, after two deadlines for a comprehensive agreement were missed.

Iran and the group known as the P5+1 - Britain, China, France, Germany, Russia and the United States - now want to conclude a political agreement by late March, with a final deadline pinning down the technical details by the end of June.

John Kerry and Javad Zarif have met many times, mostly in European cities, as they seek to thrash out the complex accord. But both men are under pressure from hardliners back home, with US lawmakers threatening to try to unleash a new wave of sanctions on Iran after March.