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Pool via Associated Press, Carlos Barria
U.S. Secretary of State John Kerry, center, and State Department Chief of Staff Jon Finer, left, meet with other members of the U.S. delegation at the garden of the Palais Coburg hotel where the Iran nuclear talks meetings are being held in Vienna, Austria, Friday July 10, 2015. Kerry urged Iran to make the “tough political decisions” needed to reach an agreement but Iranian Foreign Minister Mohammad Javad Zarif accused major powers on Friday of backtracking on previous pledges and throwing up new "red lines" at nuclear talks.

VIENNA — Nuclear negotiations with Iran appeared back on track Sunday after blowing through three deadlines, with U.S. Secretary of State John Kerry dropping warnings they could go either way but expressing hope that nearly a decade of international efforts could soon result in a historic deal.

En route to Mass at Vienna's gothic St. Stephens Cathedral, Kerry said twice he was "hopeful" after a "very good meeting" Saturday with Iranian Foreign Minister Mohammad Javad Zarif, who had Muslim services Friday.

Kerry noted that "a few tough things" remain in the way of agreement but added: "We're getting to some real decisions."

He spoke ahead of Monday target date for a pact meant to impose long-term, verifiable limits on nuclear programs that Tehran could modify to produce weapons. Iran, in return, would get tens of billions of dollars in sanctions relief.

The nuclear talks are now in their 16th day and have been extended three times since the first deadline of March 31 for the current round was missed. The mood among negotiators has turned more somber each time a new target date was set.

As the weekend approached, Kerry declared the talks could not go on indefinitely and warned that the U.S. could walk away from the negotiations.

But in another sign that a deal could soon be sealed, Russian news agencies reported that Russian Foreign Minister Sergey Lavrov planned to arrive in Vienna on Sunday evening. Most other foreign ministers of the six nations negotiating with Iran already are in the Austrian capital and in position to join Kerry and Zarif for any announcement of an agreement.

Diplomats familiar with the talks say most of the nuts and bolts of implementing the deal have been agreed upon. But over the past week issues that were previously on the back burner have led to new disputes. Among them is Iran's demand for a lifting of a U.N. arms embargo and its insistence that any U.N. Security Council resolution approving the deal be written in a way that effectively frames previous resolutions criticizing its nuclear activities as illegal.

Despite Kerry's relatively upbeat take, comments by Iran's supreme leader suggested that Tehran's mistrust of Washington would persist no matter what the outcome of the talks.

Iran's state-run Press TV cited Ayatollah Ali Khamenei on Saturday as calling the U.S. an "excellent example of arrogance." It said Khamenei told university students in Tehran to be "prepared to continue the struggle against arrogant powers."

Even if Khamenei isn't signaling that the talks have failed, his comments appeared to be a blow to U.S. hopes that an agreement will lead to improved bilateral relations that could translate into increased cooperation in a common cause— the fight against Islamic State radicals.

Zarif had hinted at just that last week, suggesting a deal acceptable to his country will open the door to joint efforts on that front.

Israeli Prime Minister Benjamin Netanyahu, a fierce opponent of what he considers a deal that is too lenient on Tehran, said Khamenei's comments showed that Western powers are "caving" into Iran even as the Islamic Republic keeps railing against them.