LAUSANNE, Switzerland — Negotiators from the United States, Iran and five other nations extended their deadline until Wednesday as they struggled to agree on a preliminary accord to limit Tehran’s nuclear program.
With a previously enunciated deadline just hours away, and after a full day of talks with most of the foreign ministers from the seven countries involved in the negotiations, an American official said that they were still working to resolve several issues.
“We’ve made enough progress in the last days to merit staying until Wednesday,” said Marie Harf, a State Department spokeswoman. “There are several difficult issues remaining.”
Another Western official, who asked not to be identified in order to discuss closed-door talks, said that the negotiations on Tuesday had been difficult.
Plans to make some kind of announcement enumerating areas of agreement after a week of intensive talks were delayed.
The March 31 deadline was established three months ago as a mechanism to determine whether there was enough political will to reach a final accord by the end of June, when an interim agreement temporarily limiting Iran’s activities expires.
Just days ago, Iran seemed intransigent on several major issues. It wanted United Nations sanctions to be lifted almost immediately, while the United States and its negotiating partners wanted any relaxation to be gradual — to ensure that Iran takes steps that would make it far harder to produce bomb-grade material in less than a year, and as it answers long-evaded questions from international inspectors.
On Monday, the State Department acknowledged that a central question, the disposition of Iran’s large stockpile of nuclear fuel, remained a subject of debate.
“The bottom line is that we don’t have agreement with the Iranians on the stockpile issue,” Ms. Harf told reporters. Western officials here suggested that the issue might be categorized a technical question and kicked down the road to the June final agreement.
“One person is missing here: It’s Ayatollah Khamenei,” a senior European diplomat, speaking on the condition of anonymity, said late on Monday, referring to the Iranian supreme leader, Ayatollah Ali Khamenei. “We don’t know what he will think of the provisions.”
Also missing is Congress, which has pledged to impose additional sanctions if a preliminary accord is not reached — a threat that may lead the Obama administration to solidify what it can get now and seek more in the next three months.
The nations involved in the talks with Iran are Britain, China, France, Germany, Russia and the United States. The Russian foreign minister, Sergey V. Lavrov was the only chief diplomat from those countries who was not here on Tuesday morning, and it had been thought that he would return if an accord was likely to be announced.
Frank-Walter Steinmeier, the German foreign minister, reinforced the sense of progress when he said Friday morning that the talks had been in something of a crisis but that negotiators seemed to have settled on “a bit of a new approach.”
But if an accord is announced on Tuesday or Wednesday, it may be as noteworthy for what it leaves out as for what it includes.
The issues the negotiators have been struggling to resolve include the pace at which United Nations sanctions would be lifted, restrictions on research and development related to new types of centrifuges, and the length of the agreement.