VIENNA, AUSTRIA —
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, the United States said Tuesday.
"We've made substantial progress in every area, but this work is highly technical and high stakes for all of the countries involved," Marie Harf, the spokeswoman for the U.S. delegation, said in a statement.
"We're frankly more concerned about the quality of the deal than we are about the clock, though we also know that difficult decisions won't get any easier with time – that is why we are continuing to negotiate," Harf's statement said.
European Union foreign policy chief Federica Mogherini said earlier Tuesday, "We are interpreting in a flexible way our deadline, which means that we are taking the time, the days we still need, to finalize the agreement," adding that there remained several difficult issues to resolve.
Earlier, negotiators seeking to reach a comprehensive agreement on Iran's nuclear program arrived at their last day of talks in Vienna with several lingering disagreements.
Teams from Iran and the group that includes the United States, Britain, China, France, Russia and Germany met for two hours in a session that began late Monday and extended into early Tuesday.
That capped what had been an intense day of negotiations as the two sides seek what they have set up as a deal that would limit Iran's nuclear activities in exchange for sanctions relief.
This round of talks, which builds on a framework agreement they reached in April, was due to end last week, but was extended by seven days.
Now, officials from both sides have signaled more time may be needed.
WATCH: VOA's Mary Alice Salinas reports from Vienna
"We don't see definite deadlines," a senior Iranian official said Monday. "We know there are certain considerations by the U.S. That doesn't mean everyone agrees."
If a deal is reached by Thursday, the U.S. Congress would have 30 days to review it and vote on whether to lift certain sanctions lawmakers have imposed. After Thursday, the review period becomes 60 days.
White House spokesman Josh Earnest said Monday that going beyond Tuesday's deadline is "certainly possible."
Iran and the group of world powers have disagreed for months on items such as the pace at which sanctions would be lifted and the level of access that U.N. inspectors would have to Iranian sites in order to monitor compliance with an agreement.
But a new issue emerged Monday, with Iranian officials saying they want a U.N. arms embargo lifted, something Western officials said they oppose.
Chris Hannas contributed to this report from Washington. Some material for this report came from Reuters and AFP.
WATCH: Nuclear deal with Iran would be complex