Reuters news agency cited Iranian Foreign Minister Mohammad Javad Zarif as saying that here will be no announcement of a deal with global powers on Tehran's nuclear program on July 12.
Zarif spoke not long after his latest meeting with U.S. Secretary of State John Kerry, on the eve of a July 13 deadline for a deal that would curb Iran's nuclear activities in exchange for relief from UN and Western sanctions.
According to Reuters, Zarif also said there would be no new extension for negotiations who have already missed three previous deadlines.
"We still have got work to do and no deal will be announced on Sunday...also there will be no extension after tomorrow," Zarif was quoted as saying.
Other officials and sources had also suggested that no deal would be announced on July 12 but that an agreement was possible on July 13.
Negotiators from Iran and the six world powers had been reportedly inching closer to a deal in recent days.
U.S. Secretary of State John Kerry had cautioned that some difficult issues persisted on the 16th straight day of talks in Vienna between ministers from Iran and the so-called P5+1 group -- the United States, Russia, China, Britain, France, and Germany.
"I think we're getting to some real decisions," Kerry told reporters on July 12.
Hours before an evening meeting with Iranian Foreign Minister Mohammad Javad Zarif that lasted about 35 minutes, Kerry said there were still "a few tough things to do" but he remained "hopeful" a final deal could be clinched.
Iranian President Hassan Rohani had also suggested negotiators were not there yet, according to the Iranian news agency ISNA.
"It might seem we have reached the top of the mountain. But no, there are still steps needed to be taken," ISNA quoted Rohani as saying. "Even if we fail...we have performed our duty."
French Foreign Minister Laurent Fabius had said he hoped the two sides have at last entered the final stages of their negotiations, although work still remained.
"I hope, I hope, that we are finally entering the final phase of this marathon negotiation," Fabius told reporters on July 12 after returning from an emergency cabinet meeting in Paris on the Greek crisis. Diplomatic sources said he cancelled a planned trip to Africa.
EU foreign policy chief Federica Mogherini said on Twitter earlier on July 12 that these were the "decisive hours."
The Associated Press news agency cited two diplomats it did not identify as saying that negotiators plan to announce on July 13 that they have reached agreement on a deal, and that a provisional agreement could be reached by late on July 12 -- but that final details were still being worked out.
In a sign that a deal could be imminent, Russian Foreign Minister Sergei Lavrov arrived in Vienna in the afternoon and Chinese Foreign Minister Wang Yi was expected as well.
But an Iranian official said it would be "logistically impossible" to conclude a deal on July 12, and Reuters news agency cited a Western official as saying Iranian and U.S. negotiators would would need time to consult their capitals once an agreement was reached.
"We are working hard, but a deal tonight is simply logistically impossible," Alireza Miryousefi, a spokesman for the Iranian delegation, said on Twitter. "This is a 100-page document, after all."
State-run Russian news agency TASS cited an unnamed diplomatic source as saying that if "today's situation makes it possible for us to reach an
agreement, we will not be able to hold the ceremony earlier than Monday."
In another indication that a deal was not likely to emerge on July 12 but was possible the next day, British Foreign Secretary Philip Hammond flew back to London but was expected to return to Vienna on July 13.
Diplomats said 98 percent of the text was finished after the flurry of bilateral and multilateral meetings on July 11.
But reports suggest two or three important issues remain to be resolved -- including how long any agreement reached in Vienna should last and the question of lifting a UN embargo on missiles and conventional weapons.
Three previous deadlines in the current round of talks -- June 30, July 7, and July 9 -- have already been missed.
Global powers and Iran are seeking a deal that would limit Tehran's nuclear activities, making it harder for Iran to develop nuclear weapons, in exchange for relief from UN, U.S., and EU sanctions that have harmed it economy.
Under a framework agreement that was reached in Lausanne, Switzerland, in April, Iran is required to cut the number of its centrifuges from more than 19,000 to just over 6,000.
It also is required to cut back its stockpile of enriched uranium, which can be used to make a nuclear bomb if enriched to a high enough level, from more than seven tons to about 350 kilograms.
The aim is to ensure that it would take Iran at least a year to acquire enough fissile material to build a nuclear bomb.
But the diplomats at the talks in Vienna have been negotiating over the issue of exactly how to implement the Lausanne guidelines.
The negotiations also have faltered over the issue of granting UN nuclear inspectors access to military sites in order to investigate suspicions that Iran sought to develop nuclear weapons in the past.
The speed of sanctions relief has been a sticking point, and there are disagreements over a longstanding an arms embargo and a ban on Iran's ballistic missile program.
Iran's nuclear program has been a source of tension for over two decades. Negotiations bore little fruit until an interim agreement was reached in November 2013.
Kerry and Zarif have met nearly every day since Kerry arrived in Vienna more than two weeks ago for what was intended to be the final phase in a negotiation process that began with an interim nuclear deal clinched in November 2013.
A deal would be the biggest breakthrough in relations between Tehran and the West since Iran's 1979 Islamic Revolution, but strong tensions are expected to persist nonetheless.
On July 11, Iranian Supreme Leader Ayatollah Ali Khamenei was quoted as telling university students in Tehran that the United States is a “perfect example of arrogance” and that they should prepare themselves “for more fight against arrogance.”
Tehran denies that it has ever sought to build nuclear weapons, saying that its nuclear program is for entirely for peaceful civilian purposes such as medical research and generating electricity.
Israeli Prime Minister Benjamin Netanyahu continued his drumbeat of criticism of a potential deal, saying an agreement would only enrich and empower Iran.
"Concessions [are in the making] even on matters that had been determined as red lines in the Lausanne deal -- which in itself is a bad deal that paves Iran's path to many nuclear bombs and gives it hundreds of billions of dollars for its terror and detention machine," Netanyahu said at a cabinet meeting on July 12.
In the United States, Republican Senate Majority Leader Mitch McConnell and other lawmakers predicted President Barack Obama could face hurdles in Congress if negotiators reach a final deal.
"This is going to be a very hard sell for the administration," McConnell said on Fox News Sunday.
If a deal is sealed, Congress will have 60 days to assess it, requiring Obama to await that review before easing some sanctions. During that time, lawmakers could try to assemble a veto-proof majority to back legislation that could impose new sanctions on Iran or keep existing ones in place.
With reporting by Hannah Kaviani of RFE/RL's Radio Farda in Vienna, AP, Reuters, AFP, RIA, and TASS