Diplomats tasked with salvaging a nuclear deal between Iran and major world powers are left playing a guessing game on when and how the talks will resume.

The negotiators have left Vienna, where rounds of negotiations have been held this year, to return home for consultations in their capitals.

Iran called for the break on Friday, though it is understood that negotiators from a number of the participating countries had wanted to press on into this week.

Diplomats from the countries involved have expressed broad agreement for the wish that the talks resume quickly, though a firm date remains elusive.

The representatives of China, France, Germany, Russia, the United Kingdom and Iran have been meeting in an effort to resume the implementation of the nuclear deal. The pact, struck in 2015, is aimed at curbing Iran's nuclear program in exchange for an easing of sanctions against the republic. The United States, which withdrew from the deal in 2018, is participating indirectly in the process unfolding in the Austrian capital.

Wang Qun, Chinese envoy to the United Nations and other international organizations in Vienna, said the talks will "resume hopefully before the end of the year".

Enrique Mora, the European Union diplomat who chairs the talks, has expressed similar optimism.

"For the eighth round, we have a lot of work ahead, a very complex task, I have to say," Mora added.

The most recent round of talks started on Nov 29, bringing the diplomats back to Vienna in another push to secure a deal for the restoration of the pact, known formally as the Joint Comprehensive Plan of Action.

From the outset, the just-concluded seventh round struggled to make headway, and was marked by the trading of accusations, said Zou Zhiqiang, a researcher at the Center for Middle Eastern Studies of Fudan University in Shanghai.

The hardening of positions has arisen from a serious lack of mutual trust between the US and Iran.

Iran has all along insisted that the US should first lift sanctions imposed on it and promise not to again withdraw from a revised agreement, said Zou, who said the US wants the removal of sanctions to be conditional on Iran's actions.

Plenty of other factors have a bearing on the outcome of the negotiations, regardless of when they resume. These include the further development of Iran's nuclear technology, and the willingness of the ruling conservatives in Iran to overcome their misgivings about the 2015 deal, the scholar said.

"None of them is willing to make concessions," Zou said of the current state of play.

Washington's allies in the Middle East have also sought to prevent the US from reaching an agreement with Iran, Zou said.

'Disappointing pause'

With the talks thrown into more uncertainty, European diplomats said it was "a disappointing pause" and that negotiators are "rapidly reaching the end of the road".

US National Security Adviser Jake Sullivan said in Washington that the negotiations are "not going well" and that the US does not yet have a path back into the deal.

However, there was one sign of progress on a related issue when Iran and the International Atomic Energy Agency reached a deal on Wednesday to reinstall cameras damaged at an Iranian site that manufactures centrifuge parts.

Wang said that after three weeks of intense negotiations and thanks to the efforts of all parties, a solid foundation has been laid for advancing the follow-up negotiations.

Zou, like Wang, holds out some hope for the diplomatic process that began in Vienna earlier this year. But he conceded that it could be "too difficult" to reach a comprehensive agreement.

Xinhua and agencies contributed to this story.