VIENNA (AP) -- Top nuclear experts from Iran and the United States huddled Wednesday on disputes that have pushed negotiations over Tehran's nuclear program into overtime. Some foreign ministers were returning to Vienna later in the day, but others were staying away, reflecting the distance still to go for a deal.

As U.S. Energy Secretary Ernest Moniz met with Iranian nuclear chief Ali Akbar Salehi, Secretary of State John Kerry and Iranian Foreign Minister Mohamad Javad Zarif took a breather from their rounds of bilateral meetings - their first break since the current round began nearly two weeks ago.

Kerry and Zarif are taking the lead in the seven-nation talks and had spoken of progress since their first meeting Saturday, adding to hopes that the original June 30 deadline would be met. But disputes over the length and kinds of limits on Tehran's nuclear program and the timing for lifting economic penalties persisted, forcing negotiators to extend, first to Tuesday and then to Friday.

Diplomats on Tuesday said that most technical questions had been agreed on. But the fact that the two top nuclear experts met twice Wednesday for at least three hours indicated problems remained that only technicians could solve.

Kerry and Zarif had been joined over the past few days by the foreign ministers of the other nations at the table - Russia, China, Britain, France and Germany. That had led to expectations that a deal could be near and awaiting a formal announcement.

But the talks broke through their second deadline in a week on Tuesday, raising new questions about the ability of world powers to cut off all Iranian pathways to nuclear weapons through diplomacy and ways of lifting sanctions on Iran. 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.

Ministers of the other nations at the table had joined Kerry and Zarif over the past few days in hopes their added diplomatic weight would clinch agreement.

President Barack 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."

"He (Obama) went out of his way last night to make it clear that he is prepared to walk away from the table and bear the consequences if the Iranians don't bend on these last few ... issues," said Sen. Chris Murphy, D-Conn.

Sen. Dianne Feinstein, D-Calif., also expressed confidence that Obama would abandon a bad deal. But asked if they had the same understanding of a "bad deal," she replied, "That remains to be seen, I don't know."

French Foreign Minister Laurent Fabius was expected back later Wednesday along with German Foreign Minister Frank-Walter Steinmeier and British Foreign Secretary Philip Hammond. But Russian Foreign Minister Sergei Lavrov and Wang Yi of China have announced no immediate plans to return, suggesting negotiators were still not close to a deal.

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.

A senior U.S. official also said the U.S. is insisting that any new U.N. Security Council resolution pertaining to Iran retain an arms ban and ballistic missile restrictions. The official demanded anonymity, in line with State Department custom.

Associated Press writers Matthew Lee in Vienna and Laurie Kellman in Washington contributed to this report.

By The Associated Press