Secretary of State
Kerry will "see if progress can be made on the issues where significant gaps remain, and assess Iran's willingness to make critical choices," Marie Harf, a department spokeswoman, said in a statement.
After five months of almost daily conversations, the six world powers are under no illusions that a meeting of their top diplomats can overcome the huge differences remaining between the two sides, officials said.
But Kerry has been lobbying his counterparts to come together in the Austrian capital, saying that a gathering of the top diplomats could make a difference.
The diplomats also want to be able to say that they made every effort to work out a deal, in case talks collapse and a "blame game" ensues over who was responsible, officials said.
Iran and the six world powers -- France, Britain, Germany, Russia, China and the United States -- have been negotiating a deal that would lift tough international sanctions on Iran's economy in exchange for restrictions aimed at preventing Tehran from gaining nuclear weapons capability.
Kerry is a believer in the power of personal diplomacy, and he has been eager to take a more active role in the Iran negotiations, which are President Obama's top foreign policy priority.
In a sign of the fading prospects for a deal, the diplomats are expected to begin discussing the sensitive issue of how to extend the deadline for talks.
U.S. officials have insisted that diplomats hadn't begun discussing an extension because they were focused on concluding the talks by the July 20 deadline. But working out an extension of the deadline would involve politically sensitive decisions, so it would make sense for the top diplomats to be present.
Iran, for example, might demand sanctions relief in addition what it got as part of the interim nuclear deal reached in November. And the six powers might want to see more restrictions on Iran's nuclear actitivities to slow its feared advance toward the capability to build nuclear weapons.
For some decisions, it may be sensible to bring in the foreign ministers, said Jofi Joseph, a former nonproliferation specialist in the Obama
The terms of an extension deal may face opposition from skeptics in
Kerry will be flying in from Afghanistan, where he arrived late Thursday to try to calm political conflict over allegations of fraud in the recent presidential election.
It is unclear how many other foreign ministers will join him in Vienna. Attendance will be "as schedules permit," Harf said.