Negotiations over Iran’s nuclear program appeared to take a sour turn Wednesday after pushing on past a key deadline, but Secretary of State John F. Kerry decided to stay in Switzerland an extra day in search of a breakthrough.

State Department spokeswoman Marie Harf said that progress had been made and that Kerry would remain “until at least Thursday morning.” But the short period appeared to reflect the difficulties in the talks between six world powers and Iran over a preliminary agreement on restricting the Islamic republic’s ability to use its civilian nuclear technology to build atomic weapons.

“We continue to make progress but have not reached a political understanding,” Harf told reporters.

The talks with Iran appeared to be on ever-more-shaky ground as the day elapsed. The White House said Iran had not made commitments about its nuclear program in Wednesday’s sessions, and Iran’s foreign minister described negotiations with the West as “always problematic.”

Though the talks continued, Germany’s foreign minister said it was possible they could collapse.

“It is clear the negotiations are not going well,” two prominent Republican senators who have been wary of an agreement — John McCain (Ariz.) and Lindsey O. Graham (S.C.) — said in a joint statement. “At every step, the Iranians appear intent on retaining the capacity to achieve a nuclear weapon.”

The Obama administration had sought a broad political framework for an agreement by Tuesday, with three additional months to negotiate the technical details. But a deadline that perhaps was intended to pressure Iran to make concessions came and went as the country’s representatives bargained hard. A temporary nuclear agreement with Iran remains in effect until June 30.

Diplomats and politicians sounded exasperated Wednesday, even as they acknowledged they were still exploring proposals to find a way out of their impasse.

In Washington, White House press secretary Josh Earnest said that the talks were productive but that there were unresolved details. He said the United States would not arbitrarily end the negotiations if they were making progress, “but if we are in a situation where we sense that the talks have stalled, then yes, the United States and the international community is prepared to walk away.”

In Lausanne, German Foreign Minister Frank-Walter Steinmeier said that new proposals would be considered but that the two sides were still far apart.

When asked whether the talks could collapse, Steinmeier told German reporters: “Naturally. Whoever negotiates has to accept the risk of collapse. But I say that in light of the convergence [of views] that we have achieved here in Switzerland, in Lausanne, it would be irresponsible to ignore the possibility of reaching an agreement.”

Steinmeier said he would reassess on Thursday morning whether to stay or return home. French Foreign Minister Laurent Fabius, who went to Paris on Wednesday morning, was headed back to the talks in Lausanne that night.

‘Mutually exclusive’

Iran’s chief negotiator, Mohammed Javad Zarif, was critical of his counterparts when he was approached by reporters as he strolled along the shores of Lake Geneva.

“I’ve always said that an agreement and pressure do not go together; they are mutually exclusive,” he said. “So our friends need to decide whether they want to be with Iran based on respect or whether they want to continue based on pressure. They have tested the other one. It is high time to test this one.”

Earlier, speaking to Iranian reporters outside the Beau Rivage Palace, where talks are being conducted, Zarif sounded weary with the approach taken by the multiple negotiating teams on the other side of the table.

“The negotiations’ progress depends on political will,” he said, according to Iran’s Mehr News Agency. “The other party’s political will has always been problematic.”

With the departure of several foreign ministers who had arrived over the weekend, Kerry was joined at the table by the British and German foreign ministers and the European Union’s foreign policy chief. France, China and Russia were represented by their ministers’ deputies.

The Obama administration and its negotiating partners are seeking an agreement that will sharply limit Iran’s ability to build nuclear weapons for at least a decade and maintain lesser restrictions in subsequent years. Iran says that its nuclear program is for peaceful, civilian purposes. It is seeking the lifting of international sanctions that have battered its economy.

The day’s negotiations started amid hopes of a preliminary agreement on at least some issues.

Iranian Deputy Foreign Minister Abbas Araghchi said he expected the talks to end late Wednesday with a statement “announcing progress.” That was quickly contradicted by diplomats from other countries.

Araghchi also offered some insight into Iran’s position on two central issues — the lifting of sanctions and the future of Iran’s research on centrifuges to enrich uranium.

“We insist on lifting of financial and oil and banking sanctions immediately,” he told Iranian state television, adding that the pace for lifting other sanctions was still being negotiated.

“We insist on keeping research and development with advanced centrifuges,” he added, referring to Iran’s desire to eventually replace its outdated centrifuges with more modern technology that enriches uranium more quickly. The United States and its negotiating partners want to keep restrictions on Iran’s nuclear research through the final years of a potential 15-year accord. They also want economic sanctions lifted more gradually.

For months, the State Department avoided the word deadline, a term that was used in Congress and the press. Officials called it a goal. In recent weeks, though, even U.S. diplomats began using the term.

“We’ve said that March 31st is a deadline; it has to mean something, and the decisions don’t get easier after March 31st,” Harf said Monday.

Some say the White House should never have adopted the “D” word.

“It was a mistake to set the March 31 deadline in the first place, because we need a positive outcome more than anyone else,” said Gary Samore, a former nuclear arms adviser to President Obama. “Naturally, the Iranians are taking advantage and playing hard ball.”

Israeli Prime Minister Benjamin Netanyahu kept up his unrelenting criticism of an agreement with Iran.

“Yesterday, an Iranian general brazenly said, and I quote, Israel’s destruction is nonnegotiable. But evidently giving Iran’s murderous regime a clear path to a bomb is negotiable,” he said in a statement from Jerusalem.

House Speaker John A. Boehner (R-Ohio), who was visiting Israel on Wednesday with a congressional delegation, said in an appearance with Netanyahu: “Regardless of where in the Middle East we’ve been, the message has been the same: You can’t continue to turn your eye away from the threats that face all of us.”

William Branigin in Washington, William Booth in Jerusalem and Karoun Demirjian in Moscow contributed to this report.

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