LAUSANNE, Switzerland — As the White House said talks on the Iranian nuclear program would go ‘‘down to the wire,’’ negotiators struggled to bridge their remaining differences in the hours before a Tuesday night deadline.
Diplomats from the six world powers conducting the negotiations with Iran held a string of meetings Monday that resembled an endurance test in their search for a preliminary agreement.
Russia’s foreign minister, Sergey Lavrov, who arrived Sunday, left for Moscow, and his spokeswoman said he would return if a deal looks realistic.
Many assessments were measured, with success and failure deemed as equally plausible.
Secretary of State John Kerry, who has been negotiating with the Iranians in Lausanne since Thursday, said that difficult issues remain on the table.
‘‘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, tweeted, ‘‘Very substantial problems remain to be solved.’’
Marie Harf, a spokeswoman for the State Department, put the odds of an agreement at 50-50. China’s foreign minister, Wang Yi, told reporters that he was ‘‘cautiously optimistic’’ and said that ‘‘positions are narrowing.’’
As the deadline loomed, 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?
The Obama administration has said any deal will extend the time Iran needs to make a nuclear weapon from the present two to three months to at least a year.
But critics object that it would keep Tehran’s nuclear technology intact.
Abbas Araghchi, Iran’s deputy foreign minister, tweeted that because the talks were ongoing, negotiators had not started drafting a document outlining 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.
Analysts said Araghchi may have been practicing some 11th-hour 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 was still open for discussion.
The White House said President Obama has been getting regular updates from the negotiating team, 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.’’
The Tuesday deadline is crucial for US 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.
An agreement, if there is one, could still be difficult to sell to a skeptical Congress and many Americans.
In Jerusalem, Benjamin Netanyahu, Israel’s prime minister, issued a statement saying that the negotiators were turning a blind eye to Iranian ‘‘aggression’’ supporting Houthi rebels in Yemen.