Representatives from Iran and six world powers are in Vienna, Austria, for a new round of talks on Iran's disputed nuclear program.
The talks, officially set to begin Wednesday, are aimed at reaching a final agreement to limit Iran's ability to produce nuclear weapons before a July 20 deadline.
Under a temporary deal reached in November, Tehran agreed to cap its nuclear activities in return for an easing of sanctions by the West.
The International Atomic Energy Agency (IAEA) indicated after a meeting with Iran on Monday that more work was needed for Tehran to fully implement a series of nuclear transparency measures by a Thursday deadline.
The West has long been concerned that Tehran's real aim is to produce an atomic weapon.
Iran denies the charge and insists its nuclear program is peaceful, aimed at producing electricity and medical treatment.