Western officials against Iranian demands to include the lifting of ballistic missile sanctions as part of nuclear deal.
Talks between Iran and Western powers to find a nuclear deal will continue until July 10, the US State Department has announced.
“To allow for the additional time to negotiate, we are taking the necessary technical steps for the measures of the Joint Point of Action to remain in place through July 10,” US State Deparment spokesman Marie Harf said on Tuesday.
Harf said “substantial progress in every area” had been made, but negotiators need time to work on the “highly technical and high stakes” details of the agreement.
“We’re frankly more concerned about the quality of the deal than we are about the clock,” she said.
The US statement comes as European Union foreign policy chief Federica Mogherini also said that negotiators “are continuing the talks for the next couple of days” beyond the July 7 deadline.
“This does not mean we are extending our deadline. I told you one week ago more or less, we are interpreting in a flexible way our deadline, which means that we are taking the time, the days we still need, to finalise the agreement,” she said.
Foreign ministers and officials from Iran and members of the P5+1 group – comprising Britain, China, France, Germany, Russia and the US – are gathered in Vienna to try to strike a deal with Iran.
Last week both sides effectively gave themselves until July 7 to finalise a deal ending a 13-year standoff by extending the terms of a November 2013 interim deal to that date.
An Iranian official previously told reporters that: “Even if we pass July 9, that will not be the end of the world, there will be another period for us to watch.”
A dispute over UN sanctions on Iran’s ballistic missile programme, as well as a broader arms embargo, are said to be among the main issues holding up the agreement before this latest self-imposed deadline.
“The Iranians want the ballistic missile sanctions lifted. They say there is no reason to connect it with the nuclear issue, a view that is difficult to accept,” one Western official told the Reuters news agency on Monday. “There’s no appetite for that on our part.”
Separately, a senior Iranian official speaking on condition of anonymity told reporters in the Austrian capital that Tehran wanted a UN arms embargo terminated as well.
Previous deadlines missed
Having agreed to an interim deal in November 2013 following the election of Iranian President Hassan Rouhani, Iran and the P5+1 missed a July 2014 deadline to get a lasting deal, and then again last November.
In April, several days late, they managed to agree on a framework accord. A target date of finalising this by June 30 was pushed back to July 7.
But by late Monday, it became clear there was still no light at the end of the tunnel despite progress in tidying up some of the details in what will be a complex and contentious agreement.
John Kerry, the US secretary of state, is under pressure to nail down the deal by Thursday in order to send it to the Republican-controlled US Congress for a 30-day review.
Under a new law, if the deal is reached after July 9, US legislators will have 60 days to vote on it, giving opponents who think the deal is too weak more chances to try to scupper the accord.
Building on the April framework, the P5+1 powers want Iran to sharply curb its nuclear programme to make any push to acquire an atomic bomb all but impossible, in return for sanctions relief.
Iran denies wanting nuclear weapons, saying its activities are purely for peaceful purposes.