THEIR COURT:

U.S. and European leaders, citing past failed openings, said they were being careful about embracing Tehran's new diplomatic overtures and the Iranian regime's expressed willingness to negotiate on the nuclear dispute.

A group including the five permanent members of the U.N. Security Council plus Germany has served as the main diplomatic channel for negotiating with Iran on its nuclear program, which the West, despite Tehran's denials, says is aimed at the production of nuclear weapons.

U.S. and European officials said they would closely monitor whether Mr. Zarif engages more substantively on the nuclear questions than his predecessors. They are also hoping he will formally respond to previous offers made by the global powers, known as the P5+1, that sought commitments from Tehran to cap its nuclear program in return for Western economic incentives and a slackening of international sanctions on Tehran.

"The question remains open" on whether Iran is serious, said a senior U.S. official involved in the diplomacy. "The ball has been in their court for some time."

A second U.S. official said the P5+1 was "waiting to see what [the Iranians] come with" on Thursday. The P5+1 and Iran will resume talks next month in Geneva, the European Union's foreign-policy chief, Catherine Ashton, said Monday.

Follow @brothersjudd