U.S. Secretary of State John Kerry boards a plane bound for Switzerland at Andrews Air Force Base on March 25, 2015. (Pool/Reuters)

The United States and Iran proceeded Thursday with a final round of talks on Iran’s nuclear future, even as they backed opposing sides in a war for control of Yemen.

Meetings, the first after a week’s break, resumed between Secretary of State John F. Kerry and Iranian Foreign Minister Mohammad Javad Zarif. U.S. Energy Secretary Ernest Moniz also attended, as did his counterpart on the Iranian side, Ali Akbar Salehi, who heads the Atomic Energy Organization of Iran, and Helga Schmid, the deputy secretary general of the European Union.

In an indication that the talks are heating up in the rush to a March 31 deadline, Iranian President Hassan Rouhani sent a letter to President Obama and the heads of the other five countries negotiating with Iran, explaining Iran's stance. He announced it on his Twitter account. The National Security Council confirmed that the letter had been passed on to the U.S. negotiating team, but its contents were not released.

Rouhani also spoke by phone with the leaders of all the nations involved in the negotiations, except for the United States.

As a backdrop to the talks, events were rapidly spiraling downward in Yemen. Saudi Arabia and its allies in the Persian Gulf launched a series of air attacks early Thursday against the Houthi rebels that Iran supports. Before he met with Zarif, Kerry took part in a conference call with foreign ministers from the Arab states in the Gulf Cooperation Council, praising their offensive, and the White House announced that the United States will provide intelligence and logistical support to their military operation in Yemen.

Graphic: Iran's potential nuclear capability

Meanwhile, Zarif told Iranian reporters in Lausanne that Shiite Iran was demanding Sunni Saudi Arabia call a halt to the airstrikes. “The Saudi-led airstrikes should stop immediately, and it is against Yemen’s sovereignty,” he was quoted saying in the Iranian news media. “We will make all efforts to control the crisis in Yemen.”

But the diplomats made a determined effort to avoid bringing the war in Yemen inside the negotiating room as they tried to reach an agreement on Iran’s nuclear program by Tuesday. A senior State Department official said the situation in Yemen had “no impact” on the nuclear talks. When reporters asked Kerry and Zarif about Yemen during a brief photo shoot ahead of the talks, both men declined to answer.

“They’re showing a willingness to compartmentalize it,” said Frederic Wehrey, a scholar with the Middle East Program of the Carnegie Endowment for International Peace. “I’m not sure Iran is willing to hitch the outcome of the talks to this somewhat peripheral conflict.” Wehrey compared the diplomacy involved to four-dimensional chess.

The Iran nuclear talks, which started over a decade ago and picked up momentum about 18 months ago when an interim agreement was signed, are reaching a culmination. The United States and Iran have both said they want to have a framework agreement by the end of March. Some of the other countries negotiating with the United States, however, have said they consider the real deadline to be the end of June, when the interim pact expires. France, in particular, has been outspoken in saying there is no need to rush because there are still large areas of disagreement.

“We’re bombing in support of Iran in Tikrit, we’re opposing them in Yemen, and we’re negotiating with them on the nuclear files,” he said. “That’s the complexity we’re dealing with. It’s strange. But the tension only manifests itself if the players allow it to.”

The goal of the talks is to impose limits on Iran’s nuclear program, which Iran says is only used for peaceful, civilian purposes. The United States, which is negotiating together with France, Britain, Germany, China and Russia, wants to block Iran’s potential to use the technology for nuclear weapons -- with the goal that it would take a year or longer to “breakout” enough fissile materials to make one nuclear warhea d. The Associated Press reported that one of several options being discussed is allowing Iran to run hundreds of uranium-enriching centrifuges at the underground Fordow site, if Iran accepts more limitations on its other sites. U.S. negotiators have said they hoped to convert Fordow to another use, leaving Natanz as the only site where uranium could be enriched for fuel and medical isotopes. Fordow is considered particularly problematic because it is an underground bunker and impervious to airstrikes should the United States or Israel determine that Iran is cheating and decide to take military action to prevent it from building a bomb.

Negotiators have stressed that the number of centrifuges is only one factor. Much of the negotiating has involved looking at variations of what would be permissible, so any concession is offset by additional restrictions. As negotiations have dragged on, the United States is prepared to accept Iran having thousands of centrifuges, instead of a few hundred as the United States demanded at the beginning of the talks.

With the deadline only days away, diplomatic efforts are ratcheting up.

Rouhani had phone conversations Thursday with the leaders of Britain, Russia and France. According to the Elysee Palace, French President François Hollande said that Iran has the right to produce civilian nuclear power but that the world wants a verifiable program to ensure Iran doesn’t get nuclear weapons. British Prime Minister David Cameron’s office said he and Rouhani agreed that a deal is possible by the end of the month. The Kremlin said Russian President Vladimir Putin and Rouhani noted progress had been made and hoped a final agreement was at hand.

On his English language Twitter account, Rouhani said an agreement depended on sanctions relief: “lifting all unjust #sanctions main step to reach a deal,” he wrote.

U.S. officials swing between confidence and caution, alternately asserting that they are closer to a deal than ever but that it could fall apart in the final stretch.

“We can see a path forward to get an agreement,” said a senior State Department official traveling with Kerry. “But that doesn’t mean we’ll get there.”

Iran’s Salehi told reporters in Lausanne that he was “optimistic” about the chances for a deal, although he has made similar analyses before, only to have the talks break off with major issues unresolved.

But in a hint that the negotiating parties are preparing for an agreement, or perhaps just getting impatient, French Foreign Minister Laurent Fabius announced that he would be coming to Lausanne on Saturday. Russian Foreign Minister Sergei Lavrov said he might come Friday. Their counterparts from the other countries, whose presence would be needed to sign an agreement, are on standby as well. And Kerry has an engagement Sunday night, although his office says the nuclear talks take priority.

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