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Ronald Zak, Associated Press
Dark clouds hang over Palais Coburg where closed-door nuclear talks with Iran take place in Vienna, Austria, Thursday, July 9, 2015.

VIENNA — Negotiations over Iran's nuclear program lurched toward another deadline on Thursday with diplomats reconvening amid persistent uncertainty and vague but seemingly hopeful pronouncements from participants.

As the talks entered a 13th day in Vienna with a flurry of closed-door meetings, Iran's President Hassan Rouhani said his nation was preparing for a "post-sanctions" era, suggesting that a deal may be in sight to curb the Iranian nuclear program in exchange for the lifting of economic sanctions. His foreign minister, meanwhile, opined on Twitter that "with mutual respect, anything possible."

U.S. Secretary of State John Kerry said in a tweet that he and the others were "continuing to discuss difficult issues" and "working diligently to see if agreement possible."

Kerry spoke by phone with Russian Foreign Minister Sergey Lavrov, who is in Russia and voiced optimism, saying he was prepared to return to Vienna.

"On the basis of the principles of gradualism and reciprocity, we have come close to a final agreement, it is within the bounds of the attainable," Lavrov said.

In another hint that the talks might soon wrap up, the White House late Wednesday issued a brief statement saying President Barack Obama had conferred with the U.S. negotiating team through a secure video call.

The last time Obama held a secure conference call with his negotiators on the road was shortly before the framework for a final accord was reached on April 2 in Lausanne, Switzerland.

The current round of talks has blown through two deadlines already and has been extended until Friday, but the Obama administration must submit an agreement to Congress before Thursday turns to Friday in Washington if it wants to avoid an extended legislative review. If the administration misses that target, the congressional review period will double from 30 to 60 days, possibly delaying the sanctions relief that the U.S. would have to give to Iran under the terms of an agreement.

Kerry, nursing a broken leg, has been in Vienna since June 26, while Iran's Foreign Minister Mohammad Javad Zarif has made one short trip to Tehran for consultations. Other foreign ministers have come and gone, however all but the top diplomats from Russia and China were present at Thursday's meetings.

When the talks missed their second deadline it raised new questions about the ability of world powers to cut off all Iranian pathways to nuclear weapons through diplomacy. Federica Mogherini, the European Union's foreign policy chief, spoke of "tense" moments, and the State Department extended the current interim nuclear arrangement with Tehran through Friday. And new difficulties also have surfaced over the past few days. Iran is pushing for an end to a U.N. arms embargo on the country but Washington opposes that demand.

Obama has said the United States would walk away from the negotiating table rather than sign a bad deal.

While congressional Republicans and Mideast allies like Israel and Saudi Arabia remain skeptical, Senate Democrats who met with Obama at the White House Tuesday night said they were confident he would only accept a "good deal."