Officials from six world powers are expected to meet Thursday for talks on persuading Iran to halt its sensitive nuclear work.
The meeting in Berlin will involve senior diplomats from Germany and the five permanent United Nations Security Council members -- the United States, France, Britain, Russia and China.
The U.S. State Department has said the meeting will not include talks on another Security Council resolution against Iran. Tehran has until May 23 to comply with the latest resolution.
The Security Council has already imposed sanctions on Iran because of its refusal to stop uranium enrichment, a process that can be used to make nuclear weapons.
On Wednesday, the United States warned that Iran may be considering leaving the nuclear Non-Proliferation Treaty.
U.S. special representative Christopher Ford told a 130-nation nuclear conference in Vienna there are signs Iran may withdraw from the treaty as North Korea did several years ago.
Ford, the chief U.S. delegate at the conference, said the U.S. is very concerned by Iran's alleged violations of the treaty.
The United States and its allies accuse Iran of trying to develop nuclear weapons, a charge Tehran denies.
Some information for this report was provided by AFP, AP and Reuters.