Secretary of State John Kerry said "tough decision" remain, but negotiators are making progress on a nuclear deal with Iran. (Reuters)

The long, grueling nuclear talks with Iran are poised to break another deadline and continue at least a little longer, as negotiators said Thursday that they have been unable to resolve the remaining issues blocking a final agreement.

Secretary of State John F. Kerry faced reporters after a day of back-to-back talks and said the United States and its five negotiating partners “will not rush, and we will not be rushed” into finalizing a deal, but he warned that they will abandon negotiations if Iran doesn’t make the “tough decisions” needed for a final agreement.

Kerry didn’t say when he would leave Vienna, but he stressed that decisions must be made “very soon.”

“This is not open-ended,” he said after walking on crutches to a lectern outside the Coburg Palace hotel where the talks are being held. “President Obama made it very clear to me last night: We can’t wait forever for the decision to be made. . . . We are absolutely prepared to call an end to this process.”

Kerry’s remarks were clearly designed as a warning to Iran. But they also follow a familiar pattern over 18 months of talks: reporting progress but bemoaning that it is not enough to clinch a deal.

U.S. Secretary of Energy Ernest Moniz, right, and Ali Akbar Salehi, head of the Iranian Atomic Energy Organization, meet Thursday at a hotel where the Iran nuclear talks are being held in Vienna. (Carlos Barria/AFP/Getty Images)

In recent days, both sides have described themselves as being closer than ever to a comprehensive agreement that would place curbs on Iran’s nuclear program in exchange for sanctions relief. But they keep stumbling over a final set of tough — possibly intractable — issues.

And they charge each other with lacking the political will to force a breakthrough.

Though many details of the closed-door talks are unknown, some sticking points have been publicly discussed.

Sanctions, for instance, remain a hurdle, even though negotiators have agreed that economic and banking sanctions will be lifted in tandem with verification from the International Atomic Energy Agency that Iran is not engaged in activities that will allow it to build nuclear weapons.

But a senior Iranian official, speaking to Western reporters Thursday night on the condition of anonymity, said Iran is not confident that sanctions will be lifted with sufficient speed by the United States and the Europeans.

The official said his mistrust existed because of the months it took to get money released from frozen Iranian bank accounts under an interim agreement reached in 2013, arguing that the money should have been freed up within days. The United States, he said, has an “obsession” with sanctions.

Addressing reporters from a balcony, Iran's foreign minister Iranian Foreign Minister Mohammad Javad Zarif says he'll stay in Vienna "for as long as necessary" to reach agreement on the country's nuclear program. (Reuters)

“I need to sense a readiness on the part of our negotiating partners,” he said. “I sense it is an attempt to keep [sanctions] rather than to let them go, as if it’s a very, very valuable asset.”

Iran also is insisting that a United Nations embargo on conventional weapons sales to Tehran be lifted, which the United States opposes. Iran is supported on this by two of the five nations negotiating alongside the United States: Russia and China, both of which hope to sell weapons to Iran.

On Thursday, Russian Foreign Minister Sergei Lavrov called for the arms embargo to be nullified as soon as a nuclear deal is reached.

In seeking to bridge the different viewpoints, diplomats are thought to be discussing ways to postpone the lifting of the arms embargo, perhaps for several years, according to diplomatic sources.

The issue illustrates the difficulty imposed by the structure of having Iran negotiate with a block of six separate countries that can have competing interests: the United States, France, Britain, Russia, China and Germany, known collectively as the P5+1.

On a typical day of negotiations, Kerry has separate meetings with his Iranian counterpart, Mohammad Javad Zarif, as well as bilateral meetings with diplomats from the P5+1 as a group. Then the P5+1 has meetings with the Iranians as a group, as well as bilateral meetings with the Iranians.

“Throughout the negotiations,” complained the Iranian official, “our friends spent more time coordinating their positions than negotiating with us.”

The Obama administration had hoped to have a final deal to submit to Congress by July 9, a deadline imposed by legislation. Under the law, missing the deadline doubles the amount of time lawmakers can spend reviewing any agreement.

Submitting a deal is a complicated, time-consuming process, involving not just providing copies of the agreement but also a written explanation of how Iran will be prevented from building nuclear weapons.

By acknowledging the talks will go on beyond midnight Thursday, the administration ensured that Congress will have 60 days for its review, instead of 30, potentially leaving more time for the opponents of a deal to marshal their arguments.

Kerry on Thursday repeatedly spoke of time running out.

“We are not going to sit at the negotiating table forever,” he said. “We also recognize that we shouldn’t get up and leave simply because the clock strikes midnight.”

U.S. officials, speaking on the condition of anonymity to discuss the talks, warned that a deal may never happen. Completing the negotiations, they said, will be akin to solving a Rubik’s Cube puzzle.

“Until the last piece clicks in, you don’t know if you can get there,” a senior administration official said this week. “You can get 90 percent of the way there, 95 and 99 percent of the way there, and you can’t get there in the end.”

Sounding fatigued, the official added: “All of us feel it would be really more than unfortunate, it would be quite a tragedy if we’ve come this far — we have really made a significant and substantial amount of progress, quite extraordinary — it would be very, very unfortunate if we could not get it done.”

William Branigin in Washington and Natasha Abbakumova in Moscow contributed to this report.