UNITED NATIONS — Wearing pinstripes and a pince-nez, Staffan de Mistura, the United Nations envoy for Syria, arrived at the Security Council one Tuesday afternoon in February and announced that President Bashar al-Assad had agreed to halt airstrikes over Aleppo. Would the rebels, Mr. de Mistura suggested, agree to halt their shelling?

What he did not announce, but everyone knew by then, was that the Assad government had begun a military offensive to encircle opposition-held enclaves in Aleppo and that fierce fighting was underway. It would take only a few days for rebel leaders, having pushed back Syrian government forces, to outright reject Mr. de Mistura’s proposed freeze in the fighting, dooming the latest diplomatic overture on Syria.

Diplomacy is often about appearing to be doing something until the time is ripe for a deal to be done.

Now, with Mr. Assad’s forces having suffered a string of losses on the battlefield and the United States reaching at least a partial rapprochement with Mr. Assad’s main backer, Iran, Mr. de Mistura is changing course. Starting Monday, he is set to hold a series of closed talks in Geneva with the warring sides and their main supporters. Iran will be among them.

In an interview at United Nations headquarters last week, Mr. de Mistura hinted that the changing circumstances, both military and diplomatic, may have prompted various backers of the war to question how much longer the bloodshed could go on.

“Will that have an impact in accelerating the willingness for a political solution? We need to test it,” he said. “The Geneva consultations may be a good umbrella for testing that. It’s an occasion for asking everyone, including the government, if there is any new way that they are looking at a political solution, as they too claim they want.”

He said he would have a better assessment at the end of June, when he expects to wrap up his consultations. That coincides with the deadline for a final agreement in the Iran nuclear talks.

Whether a nuclear deal with Iran will pave the way for a new opening on peace talks in Syria remains to be seen. Increasingly, though, world leaders are explicitly linking the two, with the European Union’s top diplomat, Federica Mogherini, suggesting last week that a nuclear agreement could spur Tehran to play “a major but positive role in Syria.”

It could hardly come soon enough. Now in its fifth year, the Syrian war has claimed 220,000 lives, prompted an exodus of more than three million refugees and unleashed jihadist groups across the region. “This conflict is producing a question mark in many — where is it leading and whether this can be sustained,” Mr. de Mistura said.

Part Italian, part Swedish, Mr. de Mistura has worked with the United Nations for more than 40 years, but he is more widely known for his dapper style than for any diplomatic coups. Syria is by far the toughest assignment of his career — indeed, two of the organization’s most seasoned diplomats, Lakhdar Brahimi and Kofi Annan, tried to do the job and gave up — and critics have wondered aloud whether Mr. de Mistura is up to the task.

He served as a United Nations envoy in Afghanistan and Iraq, and before that in Lebanon, where a former minister recalled, with some scorn, that he spent many hours sunbathing at a private club in the hills above Beirut. Those who know him say he has a taste for fine suits and can sometimes speak too soon and too much, just as they point to his diplomatic missteps and hyperbole.

They cite, for instance, a news conference in October, when he raised the specter of Srebrenica, where thousands of Muslims were massacred in 1995 during the Balkans war, in warning that the Syrian border town of Kobani could fall to the Islamic State. In February, he was photographed at a party in Damascus, the Syrian capital, celebrating the anniversary of the Iranian revolution just as Syrian forces, aided by Iran, were pummeling rebel-held suburbs of Damascus; critics seized on that as evidence of his coziness with the government.

Mouin Rabbani, who served briefly as the head of Mr. de Mistura’s political affairs unit and has since emerged as one of his most outspoken critics, said Mr. de Mistura did not have the background necessary for the job. “This isn’t someone well known for his political vision or political imagination, and his closest confidants lack the requisite knowledge and experience,” Mr. Rabbani said.

As a deputy foreign minister in the Italian government, Mr. de Mistura was tasked in 2012 with freeing two Italian marines detained in India for shooting at Indian fishermen. He made 19 trips to India, to little effect. One marine was allowed to return to Italy for medical reasons; the other remains in India.

He said he initially turned down the Syria job when the United Nations secretary general approached him last August, only to change his mind the next day, after a sleepless, guilt-ridden night.

Mr. de Mistura compared his role in Syria to that of a doctor faced with a terminally ill patient. His goal in brokering a freeze in the fighting, he said, was to alleviate suffering. He settled on Aleppo as the location for its “fame,” he said, a decision that some questioned, considering that Aleppo was far trickier than the many other lesser-known towns where activists had negotiated temporary local cease-fires.

“Everybody, at least in Europe, are very familiar with the value of Aleppo,” Mr. de Mistura said. “So I was using that as an icebreaker.”

The cease-fire negotiations, to which he had devoted six months, fell apart quickly because of the government’s military offensive in Aleppo the very day of his announcement at the Security Council. Privately, United Nations diplomats said Mr. de Mistura had been manipulated. To this, Mr. de Mistura said only that he was “disappointed and concerned.”

Tarek Fares, a former rebel fighter, said after a recent visit to Aleppo that no Syrian would admit publicly to supporting Mr. de Mistura’s cease-fire proposal. “If anyone said they went to a de Mistura meeting in Gaziantep, they would be arrested,” is how he put it, referring to the Turkish city where negotiations between the two sides were held.

Secretary General Ban Ki-moon remains staunchly behind Mr. de Mistura’s efforts. His defenders point out that he is at the center of one of the world’s toughest diplomatic problems, charged with mediating a conflict in which two of the world’s most powerful nations — Russia, which supports Mr. Assad, and the United States, which has called for his ouster — remain deadlocked.

R. Nicholas Burns, a former State Department official who now teaches at Harvard, credited Mr. de Mistura for trying to negotiate a cease-fire even when the chances of success were exceedingly small — and the chances of a political deal even smaller. For his efforts to work, Professor Burns argued, the world powers will first have to come to an agreement of their own.

“He needs the help of outside powers,” he said. “It starts with backers of Assad. That’s Russia and Iran. De Mistura is there, waiting.”