Nuclear talks between Iran and six world powers passed the negotiators’ self-imposed Tuesday night deadline last night to produce the outline of an agreement and will be extended by at least a day, the United States said.

State Department spokeswoman Marie Harf said enough progress had been made to warrant an extension, although there still were “several difficult issues” to bridge. Secretary of State John Kerry who had planned to leave the talks yesterday will remain until today, she said.

Diplomats said China’s Foreign Minister had left the talks to return to Beijing and would be represented by his deputy. US officials said they were prepared to continue to negotiate into Wednesday (today) if it could lead to a framework accord.

An Iranian negotiator, meanwhile, said his team could stay “as long as necessary” to clear the remaining hurdles.

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.

But he said that the sides were working to produce a text with few specifics, accompanied by documents outlining areas where further talks were needed.

“If it’s necessary – and, when I say if it’s necessary I mean if it’s midnight and a deal has not been reached but the conversations continue to be productive – we’ll be prepared to continue the talks into tomorrow,” he told reporters. “If we are making progress towards the finish line, than we should keep going.”

Officials said earlier yesterday they hoped to wrap up the talks by the deadline with a joint general statement agreeing to start a new phase of negotiations to curb Iran’s nuclear programme. That statement would be accompanied by more detailed documents that would include technical information on understandings of steps required on all sides to resolve outstanding concerns.

Those documents would allow the sides to claim that the new phase of talks is not simply a continuation of negotiations that have already been twice extended since an interim agreement between Iran and the so-called P5+1 nations – the United States, Britain, France, Germany, Russia and China – was concluded in November 2013.

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

After six-days of intense negotiations in the Swiss town of Lausanne, though, obstacles remained on uranium enrichment, where stockpiles of enriched uranium should be stored, limits on Iran’s nuclear research and development and the timing and scope of sanctions among other issues, according to negotiators.

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

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