EU foreign policy chief Catherine Ashton said Sunday a final accord on Iran's nuclear program cannot be guaranteed, during a landmark visit that underscores a thaw in Tehran's ties with the West.

Ashton is in Tehran on an official visit that comes after Iran signed a preliminary deal in November with world powers under which it agreed to curb its disputed nuclear activities in exchange for sanctions relief.

The breakthrough was made possible after last year's election of relatively moderate Iranian President Hassan Rouhani.

The so-called P5+1 - UN Security Council permanent members Britain, China, France, Russia and the United States plus Germany - hopes to reach final agreement by July 20, when the initial pact is due to expire.

"This interim agreement is really important but not as important as a comprehensive agreement (which is) ... difficult, challenging, and there is no guarantee that we will succeed," Ashton told a joint news conference in Tehran with Iranian Foreign Minister Mohammad Javad Zarif.

The trip is the first to Iran by a European Union foreign affairs chief since 2008, thanks to the November deal.

The next high-level talks are scheduled in Vienna on March 17 and will be followed by more rounds until July.

On a visit which winds up on Monday, Ashton also met with Rouhani as well as other senior officials.

Zarif, for his part, said Iran held up its end of the bargain and it was up to the other side to finalize the accord. Zarif said such an agreement would need to "respect the rights of Iranian people and serve national interest without ambiguities."

Ashton's visit has been billed by one European diplomat in Tehran as a "goodwill gesture from the EU."

But Israeli Prime Minister Benjamin Netanyahu criticized it Sunday.

"I'd like to ask her if she asked her Iranian hosts about the weapon delivery to the terror groups, and if she didn't ask, why not?" he said in reference to a ship the Israeli military intercepted allegedly transporting arms from Iran to Gaza.

AFP