By Paul Richter, Tribune Washington Bureau (TNS)

LAUSANNE, Switzerland — Iranian President Hassan Rouhani wrote a letter Thursday to President Barack Obama and leaders of the other five countries engaged in talks aimed at a nuclear deal, urging them to overcome differences so that an agreement can be reached by the end of the month.

Rouhani, who didn’t disclose the full contents of the letter, also raised the Saudi Arabian air attacks on Iranian-supported rebels in Yemen, an action Iran has strongly criticized. He said on Twitter that he had condemned the attacks, contending they are only “exacerbating the crisis” in a country fighting a many-sided civil war.

But Rouhani did not suggest that dispute would stand in the way of a nuclear deal, making clear, instead, that he believes the talks can reach a deal that would remove sanctions on Iran’s economy if it accepts curbs aimed at preventing it from gaining nuclear weapons capability.

The negotiators are seeking to reach the outline of a deal by Tuesday and to complete a detailed, comprehensive agreement by June 30.

Rouhani also spoke by phone with French President Francois Hollande, British Prime Minister David Cameron, and Russian President Vladimir Putin.

Rouhani hinted on Twitter that France and Britain had been pressuring Iran for concessions. He said he made the point to them that France and Britain should be “preparing for future cooperation” rather than “resorting to pressure and opposition.”

Rouhani pressed Iran’s top talking point, which is that it will only accept a deal if the six countries on the other side of the table — the United States, Britain, France, Germany, Russia, and China — agree to quickly drop sanctions.

“Lifting all unjust #sanctions main step to reach a deal,” he wrote on Twitter.

A spokeswoman for Cameron said the prime minister had told Rouhani that Iran “needs to recognize that there are concerns held by the wider international community about whether Iran’s nuclear program is being developed for peaceful purposes.”

The White House declined to comment on the contents of Rouhani’s letter but did not dispute his account.

The letter came as U.S. Secretary of State John F. Kerry and Iranian Foreign Minister Mohammad Javad Zarif resumed negotiations Thursday morning.

U.S. officials said last week’s five-day negotiating session, which ended Saturday for a three-day break, had made more progress than any previous round. They said they view March 31 as a firm deadline for the first-phase agreement they are seeking to reach.

“We very much believe we can get this done by the 31st,” a senior administration official told reporters on Kerry’s plane Wednesday night. “We see a path to do that.”

At the same time, the official acknowledged that the talks, which missed two deadlines last year, could stall again.

Though negotiators have tentatively resolved a series of thorny issues, U.S. officials indicated that a key question — how long the deal should last — has not been finally settled. An official, who declined to be identified under ground rules set by the State Department, said conversations on the subject are “ongoing.”

“All of us want as long a duration as is possible,” the official said. “The question is what is realistic — what (restrictions) should be in place for what length of time.”

Officials have said that restrictions on Iran would begin to be eased after a number of years. But some curbs, as well as close monitoring and inspections, would continue indefinitely, U.S. officials have promised.

Officials suggested that Thursday’s meeting could be crucial, because they are waiting to see if the Iranian leadership, during consultations with Zarif over the weekend, accepted the six powers’ proposals for resolving other outstanding issues.

After this meeting, “we will have a much better sense of where Iran is,” the official said.

The officials promised that the first-stage agreement — sometimes referred to as a “framework” or “political understanding” — would include enough details to allow the public to judge its value.

It has been unclear how specific the agreement would be. Iran wants no written document issued until the entire deal is complete, while Congress is demanding a detailed understanding of how the negotiators intend to resolve all important issues.

Advocates for the deal in the United States have been urging the White House to release as many details as possible so that supporters can defend it against critics in Congress and elsewhere.

“We will need to communicate as many specifics as possible to the public in some form or fashion,” an official said.

Obama this week promised that the public would be able to “lift up the hood” to see what’s in the agreement.

The officials said it was still unresolved whether the agreement would be described in a written statement, public remarks or both.

The officials said the interim nuclear agreement of November 2013, which temporarily restricts Iran’s nuclear program in exchange for limited sanctions relief, would remain in effect if the group doesn’t meet the March 31 deadline. However, they said the administration would “evaluate” what course it would take next if negotiators fell short again, for the third time.

The deadline issue is sensitive for the administration because it is under pressure to show concrete progress or face congressional action that the administration fears could sink the negotiations. The administration could theoretically continue talking until mid-April, because Congress doesn’t return from a two-week recess until April 11.

French officials have been urging the group not to worry about the March 31 deadline, because setting a date in that way gives Iran added negotiating leverage. A senior U.S. official said the six powers were united on overall strategy, though there are tactical differences between them.

A U.S. official described the plans for the talks as “incredibly fluid.” The U.S. negotiators’ schedule has no details for the next week, but says only “negotiations.”

Kerry, addressing the deal’s critics in remarks in Washington on Wednesday, warned that if the United States abandoned a deal that other world powers consider reasonable, the sanctions regime would collapse, and the Iranian nuclear threat would sharpen.

“The talks would collapse,” he said. “Iran would have the ability to go right back to spinning its centrifuges and enriching (uranium) to the degree they want.”

(c)2015 Tribune Co., Distributed by Tribune Content Agency, LLC

Photo: Secretary of State John Kerry meets with Iranian Foreign Minister Javad Zarif in Geneva on January 14 for a bilateral meeting to provide guidance to their negotiating teams before their next round of discussions, which begin on January 15. U.S. Mission/Eric Bridiers via Flickr