Escalating Iranian demands at the nuclear talks in Vienna finally reached a red line U.S. negotiators couldn't cross.
Iranian negotiators have been demanding that a U.N. arms embargo be lifted as a condition of agreeing to limit their country's nuclear program. And they've secured the backing of Russia, creating a breach in the unity of the six world powers negotiating on the other side.
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In response, Secretary of State John Kerry publicly threatened Thursday to walk away from the talks even as he and other negotiators said they were very close to a final agreement, saying "difficult decisions" need to be made "very soon."
"If the tough decisions don't get made, we are absolutely prepared to call an end to this process," he told reporters.
Iranian Foreign Minister Mohammad Javad Zarif meanwhile played his own game of brinksmanship, tweeting that his side was in no hurry.
"We're working hard, but not rushed, to get the job done. Mark my words: you can't change horses in the middle of a stream," he tweeted.
Iran and the P5+1 countries — the United States, Britain, France, Germany, Russia and China — agreed April 2 on a framework for a final deal, but fleshing out the details has proved elusive over the past two weeks of talks, which have already slipped past two self-imposed deadlines and appears ready to miss a third on Friday.
Late Thursday, negotiators from the P5+1 were working through the rift caused by Iran's demand to lift the U.N. arms embargo.
"The arms embargo on Iran must be one of the first sanctions to be lifted," Russian Foreign Minister Sergei Lavrov said Thursday, adding that it would help Iran fight terrorism more effectively.
Western negotiators have balked at the idea, and lawmakers from both parties in Washington say it's a potential deal-killer because they see Iran's ruling Shiite Muslim theocracy as the world's principal terrorist threat. After Moscow broached the idea in April in conjunction with announcing plans to complete the sale of advanced surface-to-air missiles to Tehran, U.S. lawmakers declared it a betrayal and asked President Obama to block the transfer.
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House Foreign Affairs Chairman Ed Royce, R-Calif., again raised the issue at an oversight hearing Thursday, and noted that Friday is Quds Day in Iran, an annual holiday where the regime's hostility to the United States and Israel takes center stage.
"I hope that the Obama administration will watch this spectacle and ask is this deal really the best it can do?" Royce said.
The Iranians also appear to be trying to renegotiate some of the provisions in the April 2 framework, and appear to have been successful at getting the P5+1 to make further concessions, most notably on sanctions relief and verification issues. This has put the U.S. negotiating team under severe pressure from lawmakers in Congress, most notably the Democrats on whom approval of a deal rests.
"There's a whole series of issues that if the Iranians aren't willing to accept the positions that the United States and our key allies have put forward that we should walk away from the deal," Democratic Sen. Chris Coons of Delaware, seen as a potential swing vote on a deal, told CNN Thursday.
This year, the Obama administration provoked a bipartisan backlash by dismissing congressional concerns about a potential deal, resulting in a law requiring President Obama to submit any agreement to Congress.
After Thursday passed with no agreement, the time limit for congressional review extended to 60 days from 30 until Sept. 8, giving potential opponents more time to sway opinion against any deal. Though the process is weighted in favor of approval, lawmakers had become increasingly concerned about the news from Vienna before Kerry delivered his threat.
"The deal's been trending negatively for some time," Senate Foreign Relations Chairman Bob Corker told the Washington Examiner.
Recounting the concessions over two years of talks that have made lawmakers increasingly skeptical, the Tennessee Republican noted: "It's a pretty hard pill to swallow to start in October of  with dismantling the program to move to managed proliferation where after year 10 they're fully able to industrialize their enrichment program. I just think that step in itself is pretty tough for people to deal with."