VIENNA (CNN) — Negotiators working to broker a deal to rein in Iran’s nuclear program on Tuesday extended the talks until the end of the week.

State Department spokeswoman Mari Harf said Tuesday morning in a statement that U.S. officials have “made substantial progress in every area” and will continue negotiating with their Iranian counterparts through Friday. This is the second time negotiators have extended the deadline, which was originally set for June 30.

Secretary of State John Kerry will remain in Vienna, where the latest round of negotiations are taking place.

“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, though we also know that difficult decisions won’t get any easier with time — that is why we are continuing to negotiate,” said Harf, who is the State Department’s Senior Advisor for Strategic Communications.

Officials on both the U.S. and Iranian sides have expressed in recent days that they are negotiating in earnest and making progress, and have downplayed concerns over reaching a specific deadline.

If the Obama administration does not submit a nuclear deal to Congress before Thursday, the amount of time Congress will have to review a deal will double, to 60 days from 30 days. That could set up a long and drawn-out fight to sway public opinion in favor or against the deal, with many Republicans already labeling the deal negotiators are crafting as one that would give Iran a path to a nuclear weapon.

Obama administration officials have insisted though that they will not waver on certain key points that could make or break a deal, most crucial of which is putting in place a rigorous inspections and verification mechanism that would allow international monitors to inspect any and all suspicious facilities throughout Iran.

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