Top US diplomat John Kerry and his Iranian counterpart Mohammad Javad Zarif began key nuclear talks in Geneva on Saturday ahead of a June 30 deadline to reach a historic deal.

Kerry and Zarif arrived simultaneously on the first floor of a leading Geneva hotel in opposite elevators and greeted each other with smiles and a handshake.

They chatted as they walked together along the corridor to the meeting room on the same floor.

Asked by a journalist, whether they expected to meet the nuclear negotiating deadline, Zarif smiled and said: "We will try." Kerry did not respond.

After an interim accord hammered out in Geneva in November 2013, Washington and Tehran are grappling with the final details of the ground-breaking agreement that would see Iran curtail its nuclear ambitions in return for a lifting of crippling international sanctions.