Nuclear negotiations between Iran and the six world powers have reached a decisive stage. The talks, that resume on Wednesday, will continue until July 20 - the agreed deadline for the completion of an agreement.
American sources have been saying there are big gaps between the Islamic Republic and world powers, likely due to the fact the Iranians have been toughening their stance in the home stretch.
US Secretary of State John Kerry warned Iran ahead of the resumption of the talks it still had to prove its nuclear ambitions were peaceful in an op-ed in the Washington Post late Monday.
In Tehran, Iranian President Hassan Rouhani kept up his bullish public stance on the negotiations, saying in a speech Western sanctions imposed to curb Iran's nuclear program were already crumbling.
The stakes are high in the Vienna talks, which seek a resolution to a more-than-decade-long standoff with Iran that has raised fears of a new Middle East war and a regional nuclear arms race.
Washington and some of its allies suspect Iran's program is designed to produce nuclear weapons - a charge denied by Iran which says it is only interested in generating electricity and other peaceful projects.
Kerry's comments came soon after diplomats said US Deputy Secretary of State Bill Burns was expected to attend the Vienna talks.
The presence of Burns, who led secret negotiations between Iran and the United States that helped yield a November interim nuclear agreement, would open up the possibility of bilateral talks between the two long-time antagonists.
In his op-ed, Kerry chided Iran by saying its "public optimism about the potential outcome of these negotiations has not been matched, to date, by the positions they have articulated behind closed doors."
"These gaps aren't caused by excessive demands on our part," Kerry wrote, adding that the six powers had "showed flexibility to the extent possible."
The top US diplomat said the July 20 deadline for a deal was "fast approaching".
Tehran, he suggested, can either seize the opportunity to do what is needed to ease concerns about its nuclear program or "they can squander a historic opportunity to end Iran's economic and diplomatic isolation and improve the lives of their people.
"There remains a discrepancy, however, between Iran's professed intent with respect to its nuclear program and the actual content of that program to date," he said.
"The gaps are very big. The Iranians have returned to their behavior from two-three years ago, of uncompromising intransigence. They do this under the assumption the world will grow tired and compromise. It seems like an agreement won't be reached by July 20. But there's a last-moment dynamic," Minister of Intelligence Yuval Steinitz told Ynet.
Steinitz is leading a high-ranked Israeli delegation that has arrived in Washington to ask the Americans to demand an agreement that would lead to Iran's dismantling of its nuclear program, rather than allow the program to continue under supervision.
"The fate of the world is on your shoulders. We have a lot of problems, but the Iranian threat could turn into an existential threat for Israel," Steinitz told his American hosts.
He noted that while Israel is worried about other conflicts in the Arab world, "all of these civil wars will end in a few years, while a nuclear Iran threatens the world for years to come."
While Israel does not sit at the negotiations table in Vienna, the United States has an interest in hearing Israel's professional opinion.
"We are having a very intense dialogue. We meet every month and sometimes twice a month with White House officials and senior intelligence officials," Steinitz said.
Israel views Iran's uranium enrichment and thousands of centrifuges as the main issue at stake.
"If Iran produces a nuclear bomb in the next five years, it'll be made of enriched uranium," Steinitz told the Americans. "And even if (Tehran) remains a threshold nation that's a year away from the bomb, the temptation to make the jump to the bomb will be greater."
Iran currently has 19,000 centrifuges. The Israeli position is that 3,000-4,000 centrifuges is still too much.
"If Iran has 3,000 centrifuges left, it would be able to produce enough fissile material for a bomb within less than a year," Steinitz noted.
The Israeli demand is to completely nuetralize Iran's capability to enrich uranium. While Israel wants to eliminate the threat rather than supervise it, the Americans believe they cannot entirely deny Iran centrifuges and the ability to enrich to 3.5%.
In Tehran, Rouhani vowed to continue trying to get sanctions lifted in order to improve Iran's industrial sector.
"Today, some of the sanctions have been removed and others will be lifted. I will say this again - the cup of sanctions has been broken and no one will be able to return to the situation that was before," he added.
He told Iranians not to worry about the nuclear talks, saying Tehran's negotiating team was very capable.
"We know it is difficult, but we will continue down this path until the end," he said.
One of Burns's goals this week may be to press the Iranian delegation to change its negotiating stance. Western officials close to the talks say Tehran has yet to agree to sufficient curbs on the scale of its uranium enrichment program, which is among the key sticking points in the negotiations.
Burns has met the Iranians twice in the last month, first in Geneva for bilateral talks, in an apparent effort to break a logjam between Tehran and major powers on the nuclear issue, and then in Vienna, where the wider nuclear talks take place.
During the latter talks, he broached the possibility of US and Iranian cooperation to try to stabilize Iraq against an onslaught by Sunni militants.
The United States along with Britain, China, France, Russia and Germany, a group collectively known as the P5+1, and Iran reached an interim pact on November 24 under which Iran won some relief from economic sanctions in return for reining in some of its nuclear activities.
Their target is to reach a comprehensive nuclear agreement by July 20, but outside analysts and diplomats are deeply skeptical they can achieve this.
The new round of talks will continue until at least July 15.
For the six powers, the overarching goal is to extend the time Iran would need to assemble an atomic bomb, if it chose to do so. To achieve this, they want Iran to cut the number of centrifuges in operation. Centrifuges are machines that purify uranium for use as fuel in atomic power plants or, if purified to a high level, weapons.
Both sides have said their goal is to have a deal by July 20 and avoid a difficult extension of the interim accord that expires then. Privately, Western diplomats say they would be willing to consider extending the interim deal and continuing talks beyond July 20 only if an agreement was clearly in sight.
Reuters contributed to this report.