Iranian Foreign Minister Mohammad Javad Zarif said Wednesday he was optimistic about a landmark nuclear agreement with world powers, saying negotiations were progressing amid "strong political will".

The comments by Iran's top nuclear negotiator on his Facebook page came on the eve of the resumption in Geneva on Thursday of talks aimed at putting into action the interim nuclear deal clinched in November.

"The nuclear talks are continuing with seriousness and a strong political will," Zarif wrote, 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 in 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.

Under the interim deal, once implemented, Iran will curb parts of its nuclear drive for six months in exchange for modest sanctions relief and a promise by Western powers not to impose new sanctions.

The 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.