Global powers seeking to hammer home a deal on Iran’s nuclear program missed another deadline Tuesday, with all sides vowing to now keep working until the end of the week.

Global powers seeking to hammer home a deal on Iran's nuclear program missed another deadline Tuesday, with all sides vowing to now keep working until the end of the week.

"We are continuing to negotiate for the next couple of days," EU foreign policy chief Federica Mogherini told reporters during a break from the negotiations in Vienna, effectively stopping the clock.

But the United States said the terms of a November 2013 interim accord under which Iran has been cutting back its stock of enriched uranium in return for sanctions relief would be extended until Friday, July 10, meaning this is the effective new deadline.

It was the fifth time since 2013 -- and the second time in this round of talks -- that the so-called P5+1 and Iran have missed their own target date to strike an unprecedented accord which has become bogged down over a number of weighty issues.

Mogherini admitted the seven nations at the negotiating table were "interpreting in a flexible way our deadline, which means that we are taking the time, the days we still need, to finalize the agreement".

But she said it was still possible to overcome the remaining differences and reach a deal to draw a curtain on a 13-year standoff with Iran.

US Secretary of State John Kerry would remain in Vienna with Mogherini and their Iranian counterpart Mohammad Javad Zarif, the US delegation spokeswoman Marie Harf said.

"We are taking these negotiations day to day to see if we can conclude a comprehensive agreement," she said in a statement.

"We've made substantial progress in every area, but this work is highly technical and high stakes for all of the countries involved. We're frankly more concerned about the quality of the deal than we are about the clock," Harf added.

After talking deep into the night Monday, foreign ministers from the so-called P5+1- Britain, China, France, Germany, Russia and the United States - met twice Tuesday without their Iranian counterparts.

They are seeking to hammer out a final accord on Iran's nuclear program in return for lifting a web of sanctions, after reaching a framework deal in April.