With the deadline for reaching an agreement between six world powers and Iran on the latter's nuclear program only a week away, deep differences still separate the two sides.
Iran and the six powers (the United States, Britain, France, Germany, Russia and China) convened in Vienna on Sunday in an attempt to reach agreement before the deadline expires.
But observers said the sides appear set to extend their talks past July 20, which would give more time to negotiate a deal that would limit the scope of Iran's nuclear programs in exchange for a full lifting of nuclear-related sanctions imposed on Tehran.
"Obviously we have some very significant gaps still, so we need to see if we can make some progress," Kerry told reporters before a meeting with European Union foreign policy chief Catherine Ashton, who is convening the talks.
"It is vital to make certain that Iran is not going to develop nuclear weapons, that their program is peaceful. That's what we are here trying to achieve."
While the U.S., Britain, France and Germany have sent their foreign ministers to the talks, as has Iran, China and Russia are sending lower-ranking officials instead. That may reflect their view that an extension is unavoidable.
Still, the most important disputes over how deeply Iran must cut its nuclear program are between Washington and Tehran, so Kerry's presence is crucial. He will be able to talk directly to Iranian Foreign Minister Mohammad Javad Zarif, who is at the Vienna negotiations.
Iranian Deputy Foreign Minister Abbas Araghchi spoke Saturday of "huge and deep" differences. But he told Iranian TV that "if no breakthrough is achieved, it doesn't mean that (the) talks have failed."
Kerry arrived in Vienna after diplomatic success in Afghanistan, where he persuaded rival presidential candidates to agree to a full audit of their recent runoff election. They also agreed to a power-sharing arrangement.
In Austria's capital, the top American diplomat has perhaps a harder task. Discussions center on the imposition of long-term restrictions on Iran's uranium enrichment and plutonium production — both materials used in nuclear warheads. In exchange, the U.S. and other powers would scrap a series of trade and oil sanctions against Tehran.
In Iran, hardliners oppose almost any concession by moderate President Hassan Rouhani's government.
And in the U.S., Republicans and Democrats have threatened to scuttle any emerging agreement because it would allow Iran to maintain some enrichment capacity.
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Outside the negotiation, regional rivals of Iran including Israel and Saudi Arabia are extremely skeptical of any arrangement that would, in their view, allow the Islamic republic to escape international pressure while moving closer to the nuclear club.
Iran says its program is solely for peaceful energy production and medical research purposes, though much of the world fears it's a covert effort toward nuclear weapons capability.
An interim deal in January effectively froze Iran's program, with world powers providing sanctions relief to Tehran of about $7 billion. They two sides also agreed to a six-month extension past July 20 for negotiations to reach a comprehensive deal if necessary.
With many issues left unresolved, officials with knowledge of the talks said an extension was seen as the most likely result. They spoke on condition of anonymity because they weren't authorized to discuss the ongoing negotiations publicly.
Kerry will also face tense talks with his European allies over separate matters. He will speak later Sunday with the three European foreign ministers about worsening violence in the Middle East, with each likely to push harder than the American for a cease-fire between Israel and Hamas.
Kerry met separately with German Foreign Minister Frank-Walter Steinmeier to discuss a spiraling espionage dispute between the close NATO partners.