VIENNA -- In a gilded room adorned with summer flowers, royal blue upholstery and the flags of Iran and the United States, Secretary of State John Kerry shook hands with his Iranian counterpart at the Palais Coburg in Vienna on Monday, meeting twice together in less than twenty-four hours to determine whether talks over Iran's nuclear program might be going anywhere.

The bilateral meetings between Iran and the US, represented here by the Obama administration's three highest-ranking diplomats, are an unprecedented public rapprochement: the two hour-long meetings were preceded only once this year by an exchange between Kerry and Iranian Foreign Minister Mohammad Javad Zarif in Geneva, where the two men, with other world powers, agreed to cap the nuclear impasse for at least six months.

That grace period ends in five days, however, and the two men have reconvened with their top aides in an effort to bridge "very significant gaps" on a crisis that has defined relations between their nations for decades. Zarif has expressed optimism in recent days, but US officials— camped out in Austria for the last several weeks— question Iran's seriousness at the negotiating table, given their public and private demands.

The US, with allies France, the United Kingdom, Germany, Russia and China, seek a comprehensive accord with Iran by July 20 that allays international concerns with its nuclear program. Many governments are convinced the program is military in nature, and accuse Tehran of violating the Nuclear Nonproliferation Treaty over an eighteen year period.

Kerry is in Vienna to gauge for himself whether Iran is truly prepared to make the concessions necessary for such a deal, the White House said on Monday. After the direct talks on Monday morning with Iran, officials from the European Union joined discussions in the afternoon.

A formal line of communication between the US and Iran first opened last September at the United Nations General Assembly, after 34 years of frozen diplomatic relations. Talks before then between Washington and Tehran were rare, and private.

First and foremost among the gaps facing negotiators are expectations over Iran's uranium enrichment capabilities. A major address delivered last week by Ayatollah Ali Khamenei, Iran's supreme leader, called for a marked increase in Iran's enrichment capacity, after world powers made clear for months they would settle for nothing less than a significant reduction of Iran's centrifuge infrastructure.

The speech, full of technical detail, came as a "surprise" to members of the Iranian delegation, according to their US counterparts.

One Western diplomat said the delegation appeared "taken aback" by Khamenei's remarks at such a sensitive time in the nuclear negotiations— just ahead of the July 20 deadline for a deal. Two Iranian sources confirmed that assessment.

In an interim deal reached last autumn, the negotiators agreed that talks may be extended by up to six months from July 20, should they all determine that progress has been made.

Kerry will report back his assessment to US President Barack Obama, the White House says, before the US chooses how to proceed.

Reuters contributed to this report.

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