Iran and world powers are set to resume talks Thursday on activating a landmark nuclear deal to rein in Tehran's nuclear programme, with Iranian Foreign Minister Mohammad Javad Zarif voicing optimism.

Iran's top nuclear negotiator said Wednesday that negotiations were progressing amid "strong political will", on the eve of the resumption of the Geneva talks aimed at putting into action an interim nuclear deal clinched in November.

"The nuclear talks are continuing with seriousness and a strong political will," Zarif said on his Facebook page, adding that hours of technical talks with experts from the so-called P5+1 group of world powers in December had produced "positive results".

On Thursday his deputy, Abbas Araqchi, will meet Helga Schmid, deputy to EU foreign policy chief Catherine Ashton whose office represents the P5+1 group of world powers in the decade-long negotiations with Tehran.

Their talks are due to last for two days.

The US State Department confirmed Wednesday that US Under Secretary of State for Political Affairs Wendy Sherman was also travelling to Geneva to attend the talks.

She would meet with both Araqchi and Schmid, the State Department said in a statement, without confirming reports from Iranian news agencies that there would be a three-way meeting.

Iranian foreign ministry spokeswoman Marzieh Afkham said the talks will focus on remaining issues "pending a political decision" before the deal can go into effect on January 20, a date mooted by both sides.

Technical experts from Iran and the EU-chaired P5+1 -- comprising the United States, China, Russia, Britain, France and Germany -- held two sessions in Geneva in mid and late December as they seek to fine-tune a deal reached on November 24 after their foreign ministers rushed to the Swiss city for marathon talks.

Under the deal, Iran is to curb parts of its nuclear drive for six months in exchange for modest relief from international sanctions and a promise by Western powers not to impose new measures against the Iranian economy, which has been battered by the embargo.

In early December, experts also held four days of talks in Vienna -- home of the International Atomic Energy Agency -- but the Iranians walked out after Washington expanded its sanctions blacklist against Tehran.

The interim deal is meant to buy time for diplomacy to clinch a lasting agreement that would allay Western suspicions that Iran is covertly pursuing a nuclear weapons capability.

Tehran denies wanting nuclear weapons but many in the international community suspect otherwise.

And neither Israel -- widely considered to be the Middle East's sole if undeclared nuclear-armed state -- nor Washington have ruled out military action.

Zarif said Iran was "very serious" about the negotiating process due to begin after the interim deal is implemented.

"We believe commitment to the Geneva deal will (allow) progress into the next difficult phase of negotiations, and make reaching a comprehensive accord quite conceivable," he said.

The talks, which hit a wall amid tensions between the West and hardline Iranian president Mahmoud Ahmadinejad, gathered pace after the election of relative moderate Hassan Rouhani, who succeeded him last August.

Amid signs of a thaw with the international community, Rouhani pledged transparency on the nuclear programme and engagement with major powers to try to remove the sanctions and thereby improve life for Iranians.