LAUSANNE, Switzerland - With stubborn disputes unresolved, negotiators extended nuclear talks between Iran and six world powers past a self-imposed Tuesday midnight deadline in an effort to hammer out the outline of an agreement.

Enough progress had been made to warrant the extension until Wednesday, State Department spokeswoman Marie Harf said, although there still were "several difficult issues" to bridge.

Secretary of State John Kerry, who had planned to leave the talks Tuesday, was remaining. And an Iranian negotiator said his team could stay "as long as necessary" to clear the remaining hurdles.

The decision came after six days of marathon efforts to reach a preliminary understanding by Tuesday midnight, drawing in foreign ministers from all seven nations at the table - Iran, the United States, Russia, China, Britain, France, and Germany.

After more than a decade of diplomatic efforts to limit Tehran's nuclear advances, the present talks already had been extended twice, demonstrating the difficulties of reaching an agreement that meets the demands of both sides.

The U.S. and its negotiating partners demand curbs on Iranian nuclear activities that could be used to make weapons, and they say any agreement must extend the time Tehran would need to produce a weapon from the current several months to at least a year. The Iranians deny such military intentions, but they are negotiating with the aim that a deal will end sanctions on their economy.

In Washington, White House press secretary Josh Earnest suggested that talks meant to produce an outline that would allow the sides to continue negotiations until the June 30 final deadline had not bridged all gaps. He said the sides were working to produce a text with few specifics, accompanied by documents outlining areas where further talks were needed

"If we are making progress toward the finish line, then we should keep going," he said.

Officials had hoped to wrap up the current talks by Tuesday night with that joint general statement agreeing to start a new phase of negotiations to curb Iran's nuclear program. That statement would be accompanied by more detailed documents that would include technical information on understandings of steps required on all sides to resolve concerns.

Those documents would allow the sides to claim that the new phase of talks would not simply be a continuation of negotiations that have already been twice extended since an interim agreement between Iran and the so-called P5+1 nations was concluded in November 2013.

President Obama and other leaders have said they are not interested in simply a third extension.

The softening of the language from a framework "agreement" to a framework "understanding" appeared due in part to opposition to a two-stage agreement from Iran's supreme leader, Ayatollah Ali Khamenei. Earlier this year, he demanded only one deal that would nail down specifics and not permit the other side to "make things difficult" by giving it wiggle room on interpretations.

But if the parties agree only to a broad framework that leaves key details unresolved, Obama can expect stiff opposition at home from members of Congress who want to move forward with new, stiffer Iran sanctions. Lawmakers had agreed to hold off on such a measure through March while the parties negotiated.