'Enough Progress': Iran Nuclear Talks Extended Into Wednesday

LAUSANNE, Switzerland — With several sticking points still to be worked out, negotiations on a nuclear deal with Iran will continue past Tuesday night's deadline into Wednesday, the State Department said.

Diplomats from Iran and the so-called P5+1 — the U.S., China, Russia, Germany, France and Britain — resumed meeting Tuesday night as they scramble to agree on the framework of an agreement that would see Iran limit its nuclear program in exchange for some sanctions relief.

As the midnight (6 p.m. ET) deadline crept closer, Marie Harf, the State Department's spokeswoman, said, "There are several difficult issues still remaining," but she added: "We've made enough progress in the last days to merit staying until Wednesday."

Negotiators had carefully avoided any mention of an "extension" as they worked on pulling together a statement that would close the round of talks but leave several key issues to be decided in the coming months.

Talks over the past week in Switzerland have intensified, with all sides swinging between cautious optimism and warning that differences remained that could derail a long-awaited deal to end a decade-long nuclear standoff.

One of the talks' biggest sticking points, according to U.S. negotiators, has been getting Iran to curtail its research and development on nuclear activity for at least 10 years but potentially longer. Another has been on the timing of sanctions relief for Iran if a deal is reached.

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Iran has been insisting on the removal of sanctions up front — though the U.S. and several of its allies have resisted, insisting they need some sort of leverage to ensure Tehran sticks to the deal.

Failure to break logjams on these key points would make it significantly harder for the Obama administration to fight off pending congressional legislation to assert control over any future deal — a move the White House has said will sabotage future negotiations to finalize the agreement with Iran.

It was not immediately clear what impact the outcome would have on the comprehensive deal's target of June 30. Iran and the six powers have twice extended their deadline for a long-term agreement after reaching an interim accord in November 2013.

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"I can't rule out that there will be further crises in these negotiations," German Foreign Minister Frank-Walter Steinmeier said told reporters Monday, noting that there had been "some progress and some setbacks in the last hours."

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He and his French counterpart, Laurent Fabius, canceled plans to depart for a meeting in Berlin to stay overnight as talks entered a critical phase.

NBC News' Andy Eckardt contributed to this report.