In theory, an extension to the high-stakes talks should not be a problem if all sides want it. But President Barack Obama would need to secure the consent of Congress at a time of fraught relations between his administration and lawmakers.
Iran, the United States, Britain, France, Germany, Russia and China included the July 20 deadline to reach a comprehensive agreement in an interim deal they reached in Geneva on Nov. 24.
The November agreement - under which Iran suspended some sensitive nuclear activities in exchange for limited sanctions relief - allowed for a six-month extension if more time were needed for a final settlement that would end sanctions on Iran and remove the threat of war.
An extension would allow up to half a year more for limited sanctions relief and restraints on Iranian nuclear work as agreed in Geneva. To avoid an open conflict with Congress, Obama would want U.S. lawmakers' approval to extend sanctions relief.
The latest round of talks in Vienna last month ran into difficulties when it became clear that the number of centrifuge enrichment machines Iran wanted to maintain was well beyond what would be acceptable to the West. That disagreement, envoys said, can be measured in tens of thousands of centrifuges.
As a result, the latest round of Vienna talks broke off last month with Iran and Western powers accusing each other of being unrealistic. While talk of an extension could be a negotiating tactic, members of both sides appeared to favor the idea.
EXTENSION A "FOREGONE CONCLUSION"
Barring a surprise breakthrough in the next round in Vienna on June 16-20, Western officials say an extension is virtually a foregone conclusion. "We're far apart," one diplomat said, and the talks will be "long and complicated."
The two sides said last month that they had intended to start drafting the text of a final agreement but the full-scale drafting did not actually begin.
French foreign ministry spokesman Romain Nadal said the priority for France was to reach a good deal rather than to rush through an agreement.
An Iranian official told Reuters: "We have to get rid of the sanctions immediately. Therefore, the talks will end when this issue is totally resolved. A few more months will kill no one." Pushing the deadline to October would be fine, he said.
However, the U.S. ambassador to the U.N. nuclear watchdog in Vienna, Joseph Macmanus, said it was good to have "ambitious deadlines" for negotiations, signalling that Washington remained committed to the agreed July 20 date.
"I think, again and again, you will hear from the U.S. ... that the focus is on reaching a comprehensive solution by July 20. Nothing wrong with an ambitious goal, nothing wrong with working toward that goal," he told reporters.
The 28-nation European Union - which groups three of the countries negotiating with Iran - said in a statement it would "spare no effort" to achieve the goal of a diplomatic solution by July 20 and "we call on Iran to do the same".
European Union foreign policy chief Catherine Ashton is coordinating the talks on behalf of the six powers.
Iran's envoy to the U.N. atomic agency, Reza Najafi, told reporters in the Austrian capital: "We believe that we can meet the deadline ... and we work toward that aim."
Tehran insists it needs to maintain a domestic uranium enrichment capability to produce fuel for nuclear power plants without having to rely on foreign suppliers.
Western governments and their allies suspect Iran seeks the ability to produce atomic weapons with enrichment technology, an allegation the Islamic Republic denies.
No one has an interest in letting the negotiations collapse and boosting the risk of war, said Gary Samore of Harvard University, who was the National Security Council's top nuclear security official in the first Obama administration.
"Although there will be strong opposition in both Washington and Tehran, I don't think either side can afford to take the blame for walking away from the table if the other side is prepared to continue," said Samore.
Failure of the talks would strengthen the position of conservative hardliners in Iran's clerical establishment against President Hassan Rouhani, a pragmatist who has sought to improve relations with the United States. The countries severed ties during a hostage crisis after the 1979 Islamic Revolution.
"Rouhani has put all his eggs in this basket. Failure of the talks means failure of reforms in Iran," an Iranian official close to Rouhani's government said.