Nuclear negotiations between Tehran and six world powers will not finish on Monday, Iranian Foreign Minister Mohammad Javad Zarif said, with diplomats reporting deadlock over a UN arms embargo and other sticking points.
The White House said significant issues remained to be resolved and Iran faced some tough decisions.
Asked if there would be a deal tonight, Zarif said, "No," ISNA reported. Separately, an Iranian diplomat told Iran's students news agency ISNA that a ministerial meeting, which Iranian media earlier reported would take place at 1900 GMT, is unlikely to happen on Monday.
Officials close to the negotiations spoke of the increasing frustation on the part of European delegates regarding Kerry's apparent unwillingness to walk away but the White House said 'genuine progress' had been made and the US negotiating team would stay in Vienna as long as negotiations remained useful.
"There -- there continue to be significant issues that remain," White House spokesman Josh Earnest said. "They're not going to sign onto an agreement until all of our concerns have been addressed. And as long as they continue to make progress in doing that, then the talks will -- will continue."
For days, Iran and six world powers have been close to a deal to give Tehran sanctions relief in exchange for limits on its nuclear programme, but Iranian officials said talks could run past their latest midnight deadline and success was not guaranteed.
There were no concrete signs of a breakthrough as diplomats in Vienna continued to struggle over issues such as UN sanctions and access to Iranian military sites.
Earlier, US Secretary of State John Kerry sat in silence when asked if the deadline might be extended or if he could rule out an extension. His Iranian counterpart, Mohammad Javad Zarif, said "there shouldn't be any extension", according to the semi-official Fars news agency, only to add: "But we can continue the talks as long as necessary."
Zarif spoke with Kerry and European Union foreign policy chief Federica Mogherini for more than an hour.
OIL PRICE FALLS
Oil markets appeared to be expecting a deal soon; crude futures fell about 1 percent on the prospect that sanctions would soon be lifted, allowing Iran to expand its sales into an already oversupplied market.
Diplomats said there were contingency plans for an announcement ceremony in the event of a deal, which would open the door to ending sanctions that have crippled Iran's economy, in exchange for at least a decade of curbs on its nuclear programme.
The Western powers in particular suspect Iran may have sought to use its civil nuclear programme as a cover to develop a nuclear weapons capability. Iran says the programme is solely for peaceful purposes.
If no agreement is clinched by midnight, the powers will need to extend the terms of an interim nuclear deal that has already been extended three times in two weeks.
Both the Americans and Iranians have said they are willing to walk away and they could also suspend the talks for a few weeks or months, though Iran has said it opposes this and Earnest said he was not aware of any plans for a break.
Among the biggest sticking points in the past week has been Iran's insistence that a United Nations Security Council arms embargo and ban on its ballistic missile programme dating from 2006 be lifted immediately if an agreement is reached.
Russia, which sells weapons to Iran, has publicly supported Tehran on the issue.
"99 PERCENT FINISHED"
Other problematic issues are access for inspectors to military sites in Iran, explanations from Tehran of past activity that might have been aimed at developing a nuclear weapon, and the overall speed of sanctions relief.
Iran's Fars news agency reported that Iran was pushing for the draft U.N. Security Council resolution under discussion as part of the deal to state explicitly that Tehran's nuclear programme is legal.
Comments from both senior Republican and Democratic senators on Sunday suggested that any final deal would also face tough scrutiny in the US Congress.
"The parts of the deal are there," said a senior official from one of the six countries. "We still need to put the finishing touches together. All sides have to decide now. It's time to say 'Yes'."
A senior Iranian official said 99 percent of the issues had been resolved, adding: "With political will, we can finish the work late tonight and announce it tomorrow."
Israeli Prime Minister Benjamin Netanyahu's point-man on the Iranian talks, Energy Minister Yuval Steinitz, told Israel's Army Radio he was expecting an agreement on Monday or Tuesday. He reiterated Israel's opposition to the deal.
"What is being drafted, even if we managed to slightly improve it over the past year, is a bad agreement, full of loopholes," he said. "If we call it by its true name, they are selling the world's future for a questionable diplomatic achievement in the present."