Dispute over UN sanctions on Iran's ballistic missile programme said to be a main issue of contention in negotiations.
The US State Department has announced that talks between Iran and Western powers to find a nuclear deal will continue until July 10.
"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 spokesperson Marie Harf said on Tuesday.
According to her, 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.
This follows European Union foreign policy chief, Federica Mogherini statement 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.
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.