LAUSANNE, Switzerland - Negotiators failed to reach an accord over Iran's disputed nuclear program again Wednesday, a day after they missed a self-imposed deadline, but extended the wearying talks for another day.

With negotiations still underway between Iran and six world powers Wednesday evening, aides said Secretary of State John Kerry would remain in Lausanne at least until Thursday morning in hopes of achieving a breakthrough.

"We continue to make progress but have not reached a political understanding," said Marie Harf, the acting State Department spokeswoman, referring to an outline of a deal that would kick the talks into a separate three-month phase.

Diplomats for Iran and a diplomatic bloc composed of the United States and five other world powers insist they could complete the outline at any time. But they have been unable to settle stubborn disputes that have defied resolution in seven straight days of intense talks.

German Foreign Minister Frank-Walter Steinmeier, who has generally been optimistic, told journalists that the group planned to press Iran to offer compromise solutions. He described the negotiations as a "struggle."

Over the last 18 months, negotiators have sought to craft a deal that would ease economic sanctions on Iran if it accepted restrictions intended to prevent it from someday building a nuclear bomb. If the two sides can agree on an outline, they will try to finish a comprehensive agreement by June 30.

Foreign ministers from France, Germany, Britain, Russia, and China joined Kerry in Lausanne over the weekend in an effort to build pressure on Iran. But the two sides could not resolve two major issues: how quickly to lift U.N. sanctions on Iran, and how many restrictions will remain in place at the end of the expected 10- to 15-year agreement.

Chinese Foreign Minister Wang Yi, French Foreign Minister Laurent Fabius, and Russian Foreign Minister Sergey Lavrov all abruptly left Lausanne early Wednesday after the talks blew past a midnight deadline. Fabius said he could return if needed.

The diplomats missed two deadlines last year, and President Obama told the U.S. team he wanted a definitive decision by March 31 on whether an agreement with Iran was possible. The other major powers had agreed, hoping it would put pressure on Iran's government to shelve some of its objections.

But Iran has stood firm on key issues, and the latest missed deadline has become a political vulnerability for the Obama administration. Many U.S. lawmakers and allies in Israel and Arab nations have sharply criticized the White House for allegedly making too many concessions to Iran in the talks so far.

One possibility is that the talks will break up in the next day or so with only a general news statement, then resume after a few days to try to work out a detailed agreement that can help the White House convince skeptics in Congress that it is making progress.