TEHRAN - Iran's foreign minister said he hoped a "roadmap" for negotiations could be reached in nuclear talks with world powers this week but added that a higher-level meeting would probably be needed.

The Geneva talks, set to take place on Tuesday and Wednesday, will be the first such negotiations since President Hassan Rouhani, a reputed moderate, took office in August. "I hope that we will be able to reach a roadmap by Wednesday but... it will probably be necessary to have a new ministerial meeting," Foreign Minister Mohammad Javad Zarif wrote on his Facebook page late on Sunday.

A first meeting between Zarif and his counterparts from the six powers took place last month on the sidelines of the United Nations General Assembly, which was accompanied by a landmark bilateral meeting between him and US Secretary of State John Kerry.

The Geneva negotiations will be between Iran's deputy foreign minister Abbas Araqchi and representatives of the P5+1 group made up of Britain, China, France, Russia and the United States plus Germany, who will be led by EU foreign policy chief Catherine Ashton.

After meeting Ashton in London on Sunday, Kerry said that the window for diplomacy with Iran was "cracking open".

But he also warned that Washington would remain wary during the negotiations as he spoke to the American Israel Public Affairs Committee summit in Washington via satellite from London.

"When President (Barack) Obama says that he will not allow a nuclear-armed Iran, he means what he says. I believe firmly that no deal is better than a bad deal," Kerry said.

Israel has repeatedly called on its US ally not to fall for "sweet talk" from the new Iranian president, insisting that actions not words are essential to ensure there is no possibility of Iran ever acquiring the capability to build an atomic bomb.

Israeli Intelligence Minister Yuval Steinitz called on Monday for the pressure of EU and US sanctions against Iran to be kept up, saying that was what was driving it to seek an agreement with the major powers.

"The Iranians are coming to dialogue... only because of the very severe economic pressure," Steinitz told reporters in Jerusalem.

Zarif will take part in the opening sessions in Geneva but Araqchi will lead the Iranian delegation during the talks.

The foreign minister wrote on his Facebook page, however, that "if necessary, I will also speak".

"We want to change the approach of the past six years which have given no results," he said.

Zarif has taken over as Iran's lead nuclear negotiator, but he has so far refused to be drawn on what Iran might offer in return for relief from EU and US sanctions, which have badly hit Iran's oil exports and its access to global banking.

"We will present our views, as agreed, in Geneva, not before," Zarif tweeted.

Araqchi, speaking to the state broadcaster on Sunday, said the "plan that will be presented by Zarif to the P5+1 countries during the opening session... has been prepared so that there can be no pretext to refuse it".

He did not give further details of the plan but said Iran would not accept any demand to hand over its stockpiles of enriched uranium.

"We will negotiate about the volume, levels and the methods of enrichment but shipping out the (enriched) material is a red line for Iran," Araqchi said.

Iran currently has 6,774 kilograms of low-enriched uranium, and nearly 186 kilograms of material enriched to 20 percent, as well as 187 kilograms of the 20 percent material converted to uranium oxide for use in fuel plates.

The 20 percent enriched uranium is the source of the greatest concern for the West and Israel, which fear Tehran could divert some of it for further enrichment towards the more than 90 percent level required for a nuclear weapon.

Iran insists that its nuclear programme is solely for peaceful purposes, and has defied repeated ultimatums from the UN Security Council to suspend all enrichment.