After another day of talks, international negotiators working on a deal to limit Iran's nuclear program said Thursday they have narrowed differences but are in no rush to get an agreement, backing off a previous commitment to finish by Friday.

"We will not rush and we will not be rushed," Secretary of State John Kerry told reporters in Vienna. "All that we are focused on is the quality of the agreement. And that is what will continue to define our work."

Kerry said "some of the tough issues remain unresolved" in spite of the progress that has been made, and hinted that the "difficult decisions" that remain need to be made "very soon" if the talks are to continue.

"We are absolutely prepared to call an end to this process," he said.

Iranian Foreign Minister Mohammad Javad Zarif tweeted that negotiators "are not rushed, to get the job done."

"Mark my words: You can't change horses in the middle of the stream."

The deal being negotiated with the United States, Britain, France, Germany and Russia would freeze Iran's nuclear program for 10 years in exchange for relief from international sanctions that have crippled its economy, along with the release of more than $100 billion in frozen assets.

One of the key remaining issues that appear to be holding up the talks is Iran's last-minute demand that the United Nations arms embargo and sanctions related to Iran's ballistic missile program be lifted. The dispute has split the negotiators. Russia, which is eager to sell advanced arms to Iran, has backed Tehran's demand for a removal of the arms embargo, but the United States and other European nations have balked at the idea.

U.S. negotiators also have been working in the shadow of skepticism from Congress and the public over whether concessions already made have the potential to thwart the basic goal of the talks, to prevent Iran from developing a nuclear weapon.

Under a law passed this year, Congress must review the deal. If lawmakers receive it by the end of Thursday, they will have just 30 days to review it. That time period extends to 60 days on Friday.

At the White House, spokesman Josh Earnest said Kerry has said that "important progress" has been made toward a nuclear deal between the West and Iran, but significant obstacles remain.

As long as the administration sees "a genuine commitment on the part of everyone sitting around the table … to try and constructively resolve the remaining sticking points," then talks will continue, Earnest said.

President Obama has confidence in Kerry, Energy Secretary Ernest Moniz and Wendy Sherman, under secretary for political affairs, who are leading the U.S. delegation in the Vienna talks.

"The president has been crystal clear" in what he expects from a final deal, Earrnest said. Obama will only sign off on an agreement that "shuts off every pathway" to Iran becoming a nuclear power.

Earnest said that even some Republicans have suggested keeping the interim agreement in place indefinitely, but that it is too soon to give up on reaching a final deal. However, he conceded "at some point it will become clear that the talks are no longer constructive."

Meanwhile, "there is no doubt that the interim agreement … has been good for the country and good for national security," Earnest said. He said it succeeded in freezing Iran's nuclear program and also has not allowed Tehran to "use diplomatic talks as cover to advance their nuclear program — something that they have done in the past."

When asked whether the White House's position will change at all now that Congress will have 60 days to review a final deal instead of the just 30 it would have had if a deal was sealed Thursday, Earnest said the administration is not "particularly concerned" about the review period's length.

The only reason it jumps to 60 days now is to accommodate the 30-day "vacation" that lawmakers have scheduled for themselves, Earnest quipped, referring to the upcoming August recess.