The United States told Iran Sunday it's time to consider extending nuclear talks, the first formal recognition by Washington that frenzied last-minute diplomacy may not be enough to seal a deal by a rapidly approaching deadline.

A senior U.S. official said that with the Monday evening cutoff date a little more than a day away, U.S. Secretary of State John Kerry proposed to Iranian Foreign Minister Mohamad Java Zarf that the two sides start discussing post-deadline talks in their latest meeting since Kerry arrived three days ago to add his diplomatic weight to the talks.

At the same time, two Western diplomats said, negotiations were continuing with Iran on trying to bridge differences on reducing Tehran's ability to make nuclear weapons to levels acceptable to Washington while giving the Islamic republic the relief it seeks from international sanctions over its atomic activities.

All three officials demanded anonymity because they weren't authorized to discuss the diplomatic twists and turns of talks that have been under a blanket of confidentiality since the sides started negotiating a comprehensive nuclear deal eight months ago.

The U.S. official said a number of options were under discussion.

"An extension is one of those options," the official said. "It should come as no surprise that we will also engage in a discussion of the options with the Iranians at some point as well."

Foreshadowing the developments, German Foreign Minister Frank-Walter Steinmeier told German television: "If there isn't quite a conclusion, we will have to search for possibilities to ensure that nothing breaks off here and the process can be continued."

Extension deal difficult

But reaching an agreement that allows for more negotiations could be difficult. Beyond assurances that the Iranians aren't just talking for the sake of winning time, the U.S. administration wants to show to congressional skeptics that there is sense to continuing the talks. That means Washington might push the Iranians to accept at least one of their demands on the table.

Should such a plan be agreed upon, talks could resume in early December when Kerry plans to return to Europe for a previously scheduled NATO foreign ministers meeting in Brussels, and an international conference in London.

Outlining some differences between Washington and Tehran in general terms, President Barack Obama said the U.S. goal is "to shut off a whole bunch of different avenues whereby Iran might get a nuclear weapon, and at the same time make sure that the structure of sanctions are rolled back step for step as Iran is doing what it's supposed to do."

"I think Iran would love to see the sanctions end immediately, and then to still have some avenues that might not be completely closed, and we can't do that," he told ABC's "This Week" in comments aired Sunday

Foreign ministers arrive

As the clock ticked down, other foreign ministers from the six negotiating powers were converging on Vienna in a concerted effort to at least get an acceptable way to extend the talks even if the deal deadline is missed.

The foreign ministers of Germany, Russia and France were already in Vienna by late Sunday. Britain's foreign secretary was en route and China's foreign minister was scheduled to arrive Monday.

Kerry, who arrived Thursday, also met with Saudi Foreign Minister Saud al-Faisal, whose country is vying for Middle East influence with Iran. Diplomats said Saud flew to Vienna from Paris solely for the briefing, and the two talked in his plane parked on the Vienna Airport tarmac.

The talks in Vienna aim for a deal that could transform the Middle East, open the door to ending economic sanctions on Iran and start to bring a nation of 76 million people in from the cold after decades of hostility with the West.

The cost of failure to reach a deal could be high. Iran's regional foes Israel and Saudi Arabia are watching the Vienna talks nervously. Both fear a weak deal that fails to curtail Tehran's nuclear ambitions, while a collapse of the negotiations would encourage Iran to become a threshold nuclear weapon state, something Israel has said it would never allow.

It became increasingly clear during a week of intensive negotiations between the top U.S. and Iranian diplomats that what officials close to the talks have been predicting privately for weeks is proving to be correct: a final deal is still too far off to hammer out by the parties' self-imposed deadline.

A European official said the possibility of securing a final agreement "seems physically impossible," echoing comments by Iranian officials.

Iran must not be allowed to set itself up as a nuclear threshold state. - Israeli Prime Minister Benjamin Netanyahu

With the deadline less than 24 hours away, the issue was one of several options for negotiations raised in Kerry's latest meeting with Iranian Foreign Minister Mohammad Javad Zarif, a senior U.S. official said on condition of anonymity.

The United States, Britain, France, Germany, Russia and China began the final round of talks with Iran on Tuesday.

Some Western officials describe two possible options for a likely rollover. Under one scenario, described as the "stop the clock option", the talks would simply break off and experts from the parties would reconvene in a few weeks for another attempt.

A lengthier option would be a formal extension into next year, adding new elements to an interim accord from last year.

U.S. Secretary of State John Kerry and Iranian Foreign Minister Mohammad Javad Zarif, right, discussed a possible extension of the negotiations, as a deadline for a nuclear deal between Iran and world powers fast approaches. (Ronald Zak/Associated Press) Tehran blames the West for the deadlock, saying it has consistently made unreasonable demands of Iran.

They have also warned that the upcoming change in U.S. Congress, where hard line Republicans will soon dominate both houses, means U.S. lawmakers may push for new sanctions on Iran, which the Obama administration has said could torpedo the talks.

The negotiations aim to end Western suspicions that Iran is seeking an nuclear bomb capability, while allowing Iran to have the civilian nuclear program it says is its right under international rules. Tehran denies seeking nuclear weapons.

In a breakthrough preliminary deal reached a year ago, the United States and European Union agreed to ease some sanctions on Iran while Tehran agreed to some curbs on its nuclear programs. But a final deal proved elusive, with the sides forced to extend an earlier deadline in July.

The main sticking points in the talks are the scope of Iran's enrichment program, the pace of lifting sanctions that have crippled Iran's economy and the duration of any deal.