Iran and six world powers missed their second self-imposed deadline to strike a final deal on Iran's nuclear issue on Tuesday, leaving the real deadline of the deal unknown.

Talks between Iran and major powers on Iran's nuclear issue will continue after Tuesday for a "couple of days", while the negotiations entered the most difficult and sensitive part of the talks, European Union foreign policy chief Federica Mogherini told reporters on Tuesday in Vienna.

The remarks of Mogherini came when several foreign ministers from six world major countries are finishing their meetings with their Iranian counterpart in the ongoing Iranian nuclear talks in the capital city of Vienna.

She said the deadline of the talks could be treated in a flexible way to give the negotiators more time to finalize the deal.

While Russian Foreign Minister Sergei Lavrov said there will be no artificial deadlines set for the ongoing nuclear talks, media reports from the United States suggested that an interim nuclear agreement between Iran and six major powers will be extended through Friday to provide more time for talks on a final agreement.

On the U.S. side, negotiators are "more concerned about the quality of the deal than about the clock", said U.S. State Department spokeswoman Marie Harf on her twitter account.

"We've made substantial progress in every area, but this work is highly technical and high stakes for all of the countries involved," said Harf.

Referring to sticking points remaining in the talks, French Foreign Minister Laurent Fabius told reporters: "As far as France is concerned, we are insisting especially on necessary limitations on nuclear development and research, sanctions and their re-establishment, and the possible military dimensions" of past Iran's nuclear research.

Officials and experts from Iran and P5 plus 1 countries are now busy discussing some eight technical issues needed to be agreed on before diplomats proceed with further steps towards a general nuclear agreement, Lavrov told reporters, noting that the "polishing" of these issues will take a day or two.

China remains confident about reaching a final deal in this round of talks. Despite of around six or seven remaining issues, "all sides realize that this is the last negotiations," said Wang Yi, the Chinese foreign minister.

Meanwhile, some foreign ministers are leaving on Tuesday, but will come back to Vienna in the coming hours or days, said Mogherini.

Iran and five United Nations Security Council permanent members plus Germany missed the previously-set June 30 deadline for a comprehensive deal. Both sides agreed to extend the deadline to July 7.

All parties have been negotiating over the past 16 months to reach a long-term deal over Tehran's disputed atomic plan.