Iran said bilateral nuclear talks with the United States were proceeding in a good atmosphere despite lingering gaps over key issues such as Tehran's uranium enrichment capacity and how fast economic sanctions should be lifted.

US and Iranian diplomats began a two-day meeting in Geneva on Monday (December 15) to pave the way for resuming broader negotiations involving Iran and six world powers there on Wednesday.

They are aimed at resolving a 12-year stand-off over Iran's disputed nuclear aspirations that has wrought heavy economic sanctions on the Islamic Republic and fears of a new Middle East war unless the dispute can be settled diplomatically soon.

Iran, the United States, France, Germany, Britain, China and Russia failed to strike a long-term deal by a self-imposed November 24 deadline. So they extended the talks for seven more months to address what they call complex technical details.

Monday's session, the first to take place in the overtime period, "lasted more than six hours and proceeded in a good ambience," said Iranian chief negotiator and deputy foreign minister Abbas Araghchi.

"There were elaborate discussions on all topics, especially sanctions. At present, issues have boiled down to very minor, fine details. Solutions exist in some cases, in others there is need for more effort and discussions," he said on Iranian state television.

"We are not in a position yet to judge the final outcome, but I can say the atmosphere of these sessions is good, although there is yet some distance to a solution."

Araghchi said Tehran would hold further discussions with the United States, its arch-adversary since its 1979 Islamic Revolution but also most important interlocutor, before resuming full-fledged talks with the six powers on Wednesday.

President Hassan Rouhani, a pragmatist whose election in 2013 revived diplomacy after years of deepening confrontation with the West, pledged on Monday to overcome hardline domestic opposition and secure a nuclear deal he said would bring an "end to oppressive sanctions against the Iranian people". - Reuters