As the White House said that talks on Iran’s nuclear program would go “down to the wire,” negotiators struggled to bridge their remaining differences in the hours before a Tuesday deadline.

With the clock ticking, diplomats from the six world powers conducting the negotiations with Iran held a string of meetings that have come to resemble an endurance test in their search for a preliminary agreement.

Russian Foreign Minister Sergei Lavrov, who arrived Sunday, left for Moscow, and his spokeswoman said he would return if a deal seems realistic.

Many assessments were measured, with success and failure deemed equally plausible.

Secretary of State John F. Kerry, who has been negotiating with the Iranians in Lausanne since Thursday, said that difficult issues remain on the table.

Graphic: Iran's potential nuclear capability

“We are working very hard to work those through,” he said Monday. “We are working late into the night and obviously into tomorrow. We are working with a view to get something done. There is a little more light there today, but there are still some tricky issues. Everyone knows the meaning of tomorrow.”

Gerard Arnaud, the French ambassador to Washington, said on Twitter, “Very substantial problems remain to be solved.”

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

Marie Harf, a spokeswoman for the State Department, put the odds of an agreement at 50-50. “There’s a chance we will get it done,” she said.

Chinese Foreign Minister Wang Yi told reporters that he was “cautiously optimistic” and saying that “positions are narrowing.”

As the deadline approached, the negotiators worked to settle some core issues: What kind of nuclear research would Iran be allowed to pursue in the final five years of a 15-year accord? When can the United Nations’ sanctions be eased? Will the sanctions be lifted or merely suspended so that they can be slapped back into place if Iran does not meet its commitments?

Abbas Araghchi, Iran’s deputy foreign minister, tweeted that because the talks were ongoing, negotiators had not started drafting a document that outlines agreed-upon principles that would guide further discussions on complex scientific details. A final agreement is to be worked out by the end of June.

The White House said that President Obama has been receiving regular updates from the negotiating team, which is led by Kerry and Energy Secretary Ernest Moniz, and it predicted that the talks would stretch into the final hours.

“I’m not going to presuppose failure,” White House spokesman Eric Schultz said. “Those negotiations are going to go down to the wire.”

In Jerusalem, Israeli Prime Minister Benjamin Netanyahu issued a statement saying that the negotiators were turning a blind eye to Iranian “aggression” supporting Houthi rebels in Yemen.

“The agreement being formulated in Lausanne sends a message that there is no price for aggression and, on the contrary, that Iran’s aggression is to be rewarded,” he said.

“The moderate and responsible countries in the region, especially Israel and also many other countries, will be the first to be hurt by this agreement. One cannot understand that when forces supported by Iran continue to conquer more ground in Yemen, in Lausanne they are closing their eyes to this aggression.”

The Tuesday deadline is crucial for U.S. negotiators, because Obama and Kerry have said that if a framework agreement is not reached by then, they will have to assess whether to continue the process. But an interim agreement, under which Iran has limited its nuclear output, does not expire until June 30. Negotiators from France, Germany, Britain, China and Russia have expressed less urgency about getting some sort of understanding outlined by midnight Tuesday.

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

An agreement, if there is one, could still be difficult to sell to a skeptical Congress — and many Americans.

“The very notion of America compromising with Iran is very difficult,” said Trita Parsi, president of the National Iranian American Council, who is in Lausanne and following the talks. “We’re dealing with what America thinks of as a Third World country, and the United States doesn’t have to compromise.

“But this is not about discussing Iran’s capitulation. It’s a negotiation.”

Parsi said that Araghchi may have been practicing some last-minute brinkmanship when he told Iranian reporters Sunday that Tehran would not send most of its stockpiles of enriched uranium to Russia.

Araghchi has made similar comments in recent months, and Tehran swiftly denied that any decision on stockpiles had been reached. A senior State Department official said that the fate of Iran’s stockpiles is still open for discussion.

“Iran has by and large accepted American demands,” Parsi said. “What it’s not accepting is what it’s getting in return. The United States has said it won’t lift sanctions upfront. But it may have to.”

William Booth in Jerusalem contributed to this report.

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Today's coverage from Post correspondents around the world