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

Ashton's official visit to Tehran 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 Iranian President Hassan Rouhani, viewed as a relative moderate who has the ear of supreme leader Ayatollah Ali Khamenei.

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 with Iranian Foreign Minister Mohammad Javad Zarif.

In an interview posted online by the European Commission late Sunday, Ashton reiterated although she had felt there was support "across the political spectrum" in Iran to move talks forward "that does not mean that we'll get an agreement".

"But I have had a real sense that people are committed to wanting to see the talks happen and that, I think, is encouraging of itself."

Her trip is the first to Iran by a European Union foreign affairs chief since 2008, thanks to the November deal that raised hopes of diplomacy resolving the nuclear issue.

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

On a visit which ends Monday and has drawn criticism from Israel, Ashton, who leads the P5+1's engagement with Iran, also met Rouhani and other senior officials.

Zarif said Iran held up its end of the bargain and it was up to the other side to finalise the nuclear accord.

"Iran is determined to reach an agreement. We have shown good faith and political will. We have done our part," Zarif said.

- Iran confident of deal -

Such an agreement would need to "respect the rights of Iranian people and serve national interest without ambiguities", he said.

Zarif expressed confidence a deal was within reach by July.

Ashton's visit had been billed by one European diplomat in Tehran as a "goodwill gesture from the EU", but Israeli Prime Minister Benjamin Netanyahu criticised it on 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 of a ship the Israeli military intercepted allegedly transporting arms from Iran to Gaza.

Israel's Intelligence Minister Yuval Steinitz said: "I would expect Catherine Ashton to cancel or at least postpone her visit to Tehran... At this time to go and speak with the Iranians as if nothing happened, is something that should be avoided."

In his talks with Ashton, Rouhani, quoted by Iran's ISNA news agency, called for closer ties with the EU, notably over energy and trade.

Both parties could also "cooperate in the fight against terrorism, drug trafficking, Afghanistan, Iraq or also Syria," he said.

Human rights was another issue on Ashton's agenda, despite the potential to upset Iranian hardliners.

Ashton said she and Zarif spoke "about the potential of human rights dialogue in the future" and said she was "proud" to have met on Saturday night with Iranian women activists on the occasion of International Women's Day.

An eight-member European Parliament group visited Iran in December and met rights lawyer Nasrin Sotoudeh and film-maker Jafar Panahi, sparking criticism from conservatives.

Sotoudeh was released from jail along with nearly a dozen other political prisoners last September, during Rouhani's charm offensive.

Also on Ashton's agenda was the conflict in Iran's ally Syria, which Zarif described as "dangerous".

The United States, other Western powers and Israel have long suspected Iran of using its civil nuclear energy programme as a cover for developing atomic weapons, a charge denied by Tehran.

There are still many outstanding sensitive issues including the scope of Iran's enrichment programme and demands that its bunkered Fordo uranium enrichment site be closed along with the Arak heavy-water reactor.