Iran and six world powers have once again extended a deadline for a deal on Tehran's atomic program. A U.S. official told Reuters on Friday that the date has been pushed to Monday to allow negotiators to work through the weekend to iron out final details.

"To allow for the additional time to negotiate, we are taking the necessary technical steps for the measures of the [interim nuclear deal] to remain in place through July 13," the official said.

Negotiators had initially set a June 30 deadline for a final agreement, but that was pushed to July 7, and then to July 10. After missing the U.S. congressional deadline on Friday, officials said they were extending Iranian sanctions relief to Monday to allow for negotiations to continue over the weekend in Vienna.

Iran and the P5+1 world powers - Russia, China, Britain, France, the U.S. and Germany - are pursuing an historic deal to prevent Iran from obtaining a nuclear weapon. In return for Iran placing verifiable limits on its nuclear program, a number of detrimental international and U.S. sanctions would be lifted, hopefully ending the 12-year dispute over Tehran's atomic program.

Iran has indicated that it is unlikely to begin winding down its nuclear program until sanction relief is provided.

"If you drive the talks into a dead end then it will be you who will be committing a strategic mistake," Iran's parliamentary speaker Ali Larijani said Friday, according to The Associated Press.  "And its outcome will not benefit you since Iran's nuclear staff are ready to accelerate nuclear technology at a higher speed than before."

Progress has been made, but several last-minute obstacles remain. Iran and Russia want a U.N. arms embargo lifted, which Western powers want to keep in place. Russia and China are reportedly anxious for the embargo to be lifted so they can sell ballistic missiles and other weapons to Tehran, reports USA Today.

Iran has also been hesitant to allow inspectors unfettered access to its military sites, and questions still remain regarding past Iranian activity suspected to be military in nature.

Iran blamed "excessive demands" and changing positions of Western nations for Friday's missed deadline, reported the Telegraph.

Hours before the deadline to present a deal to U.S. Congress was missed, Iranian foreign minister Mohammad Javad Zarif spoke out against several countries.

"Unfortunately we have seen changes in the position and excessive demands ... by several countries," Zarif told the Iranian television Al-Alam, according to the Telegraph.

Zarif added that each country in the group has "different positions which makes the task even harder."

"We are making progress, it's painfully slow," he told reporters before leaving Vienna, according to Reuters.

Since Friday's deadline was missed, the U.S. Republican-led Congress will now have 60 days, rather than 30, to review a final accord, which the Obama administration worries may jeopardize the deal. President Obama cannot waive sanctions on Iran until that congressional review is complete, according to AP.