Political directors of the five permanent members of the UN Security Council and Germany — the so-called P5+1 — were holding a one-day meeting in the Swiss city with the Iranians, mediated by the European Union.
An EU spokeswoman in the Swiss city told AFP the talks had begun, without giving further details.
No announcements are expected after the discussions conclude.
The US and Iranian delegations met on Monday and Tuesday in Geneva in preparation for the multilateral talks, led by Acting Deputy Secretary of State Wendy Sherman and Iran's deputy foreign minister Abbas Araqchi.
The P5+1, which comprises China, the United States, France, Russia, Britain and Germany, last held formal talks with the Iranians last month in Vienna.
They failed to meet a November 24th deadline for a comprehensive deal with Iran on reining in its nuclear programme in exchange for an easing of crippling international sanctions.
All parties agreed to give themselves seven more months — until June 30 — to strike a deal, although they said they hoped to have the broad outlines hammered out by March.
A final agreement is aimed at ensuring Tehran will never develop nuclear weapons under the guise of its civilian activities.
Iran denies that it is seeking the bomb and maintains its nuclear activities are for solely peaceful purposes.