Negotiations over Iran’s nuclear program blew past a Tuesday deadline, U.S. officials said, with diplomats struggling to reach con­sensus on the difficult issues of sanctions, enrichment research and future limits.

The Obama administration had committed itself to reach a broad political agreement with Iran and a group of world powers by March 31, with three additional months to nail down many complex details.

The decision to keep talking suggests negotiators believe an accord in some form is still possible. Secretary of State John F. Kerry, who had been planning to return to Washington if there was no agreement by midnight on Tuesday, switched gears and announced shortly before 9 p.m. local time that he would be staying at least one more day.

“We’ve made enough progress in the last days to merit staying until Wednesday,” said the State Department’s acting spokeswoman, Marie Harf, adding: “There are several difficult issues still remaining.”‎

The continuation of talks does not signal an end to the drama. White House press secretary Josh Earnest said that President Obama’s patience with the negotiations is not limitless.

“It’s time for Iran to make the serious commitments that they know the international community is expecting them to make to reach an agreement,” he said.

He noted that an interim agreement in which Iran limited its nuclear output while negotiating a final deal remains in effect until the end of June. But Earnest added: “If we’re not able to reach a political agreement, then we’re not going to wait all the way until June 30th to walk away.”

The extension was surprising, partly because State Department officials had been adamant that the March 31 deadline they set when the interim agreement was extended in November was firm.

In fact, the deadline was mostly about American politics. The Obama administration is trying to get an agreement with Iran before congressional critics have a chance to pass bills requiring their approval of any nuclear deal or imposing more sanctions on the country. Several bills are pending that would give Congress the option to reject a final accord.

Few lawmakers reacted publicly Tuesday to the latest developments in the talks. But Sen. Tom Cotton (R-Ark.) said that the prolonging of talks into an extra day, “in the face of Iranian intransigence and duplicity, proves once again that Iran is calling the shots.” He also predicted that Obama would make “further concessions” to Tehran.

Sen. Mark Kirk (R-Ill.), author of a bill that would impose new sanctions on Iran, said that Congress should vote on that measure “instead of another extension of nuclear talks.”

But Congress is on break until April 14, and some members have urged the administration not to be a prisoner of an artificial deadline.

In Lausanne, where the talks are being held, U.S. negotiators led by Kerry held a grueling schedule of meetings Tuesday with diplomats from Iran, Britain, France, Germany, China and Russia, starting shortly after sunrise and stretching late into the night. Chinese Foreign Minister Wang Yi departed the talks to return to Beijing. But Russian Foreign Minister Sergei Lavrov, who left Monday saying he would return when a deal looked “realistic,” flew back from Moscow to rejoin the negotiations. He pronounced the prospects for an agreement “good.”

The talks, which began in 2003 but picked up momentum a decade later, have already produced tentative accords on dozens of issues. But negotiators have cautioned repeatedly that nothing is truly settled until agreements are reached on all issues.

The United States and its five negotiating partners are aiming for a deal that will block Iran’s ability to build nuclear weapons for at least a decade, with diminishing restrictions in later years. For Iran, which insists it wants to use nuclear technology only for peaceful, civilian purposes, the primary goal is to get international sanctions lifted. The measures have been disastrous for its economy.

[A framework? A deal? The semantics of the talks.]

Despite progress at almost every stage of the talks, the final weeks have been consumed by negotiations over differences that are the hardest to bridge.

Iran wants sanctions lifted quickly, while the world powers are holding out for a more gradual easing. The United States and its allies want restrictions to continue in the final five years of a 15-year accord that would monitor and limit Iran’s nuclear program, with research done on the country’s outdated uranium-enrichment centrifuges. But Iran wants to be free of restraints so it can introduce newer technology that enriches uranium more quickly.

The two sides are also at odds over the fate of Iran’s stockpiles of enriched uranium, with Tehran balking at the idea of sending the material outside the country. U.S. officials have said other options are being considered, so long as they maintain a one-year “breakout” period during which Iran would not be able to amass enough material to build a nuclear bomb. Its current breakout time would allow it to build a bomb in an estimated two to three months.

As talks dragged on, the Obama administration set down the March 31 deadline for a broad “framework” agreement that would guide three more months of negotiations on “technical details.” If an agreement is reached, many thorny and substantive issues are likely to be pushed into the technical phase. A comprehensive accord is by no means a certainty.

[Fact-checking Obama’s reference to “unprecedented” nuclear inspections]

It’s still unclear what form a preliminary agreement might take. Iran is opposed to what would be in effect two separate agreements. Its supreme leader, Ayatollah Ali Khamenei, has insisted on only one, the final accord, so the United States and its allies cannot “make things difficult” by challenging interpretations.

U.S. officials, who prefer a highly detailed agreement, have said they may come up with a general statement outlining the principles for continued negotiations but provide more specifics in a separate document issued simultaneously. Whatever word is used to characterize any deal, the fact that some key issues remain unresolved could make it more difficult for the administration to fend off congressional votes on additional sanctions.

Even as the talks were going on, Israeli Prime Minister Benjamin Netanyahu kept up his criticism of an agreement with Iran.

“The greatest threat to our security and to our future was and remains Iran’s effort to arm itself with nuclear weapons,” he said. The agreement being negotiated, he continued, “paves the way to this outcome.”

Netanyahu said Iran would be left with its underground research labs, thousands of centrifuges and a heavy-water reactor that can be used to produce plutonium, another pathway to making a bomb.

“Iran’s breakout time to have the tools to make a nuclear weapon won’t be years, as was said in the beginning,” he said. “In our estimate, it will be reduced to perhaps a year, most likely much less than that.”

Karen DeYoung and Sean Sullivan in Washington and William Booth in Jerusalem contributed to this report.

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