at the UNITED NATIONS — Kofi Annan’s plan to curb the violence in Syria hit a dead end this month, another casualty of an escalating conflict that shows no signs of abating.

But Annan’s failure may have taken another toll — on the reputation of a career peacemaker and, by extension, on confidence in the power of diplomacy to resolve what is turning out to be one of the most intractable crises to grow out of the Arab Spring.

World leaders and U.N. experts have commended Annan for showing the courage to take on what they term a diplomatic “mission impossible.” But among Syrian activists and Arab critics, he has been vilified as a shill for President Bashar al-Assad. Many of those close to Annan fear that his legacy is at risk and that it is time for him to confront reality and step down from his position as special envoy.

“I feel very sorry for Annan, not because I’m an old loyalist but because he has been dealt this hand,” said a senior U.N. official involved in Syria diplomacy, speaking on the condition of anonymity in order to comment candidly. “If I were him, I would be seriously thinking about resigning. Why keep banging your head against a brick wall when you’ve become a hated figure in social media around the world? And for what?”

That bitter assessment reflects a conviction among sympathetic observers that Annan is set to be a scapegoat to shoulder the blame for key powers, including the United States and its Arab and European allies, that have been unable to persuade or compel Assad to cease the killing, as well as Russia and China, which have blocked efforts at the United Nations to punish Assad for his conduct.

But critics say Annan has allowed himself to be used by Assad and the Syrian government’s Russian and Chinese patrons in a naive effort that has allowed authorities in Damascus to buy time to crush the opposition.

“There is a kind of happy convergence between Kofi’s willingness to try a thing that may make him look naive and the world’s wish to have him try this because it doesn’t have anything more effective and forceful that it is prepared to do,” said James Traub, author of a book on the former U.N. chief, “The Best Intentions: Kofi Annan and the UN in the Era of American World Power.”

‘The secular pope’

In March, after appeals from Secretary of State Hillary Rodham Clinton and other key players, Annan was asked by U.N. Secretary General Ban Ki-moon to come out of retirement and serve as a mediator in Syria.

A Nobel Peace Prize laureate — dubbed “the secular pope” by some — Annan seemed a natural choice, in part because he enjoyed the trust of all the key powers.

While Annan has been associated with some of the United Nations’ greatest failures, having headed up the U.N. peacekeeping department during the mass killings in Bosnia and Rwanda, he also racked up diplomatic achievements such as guiding East Timor to independence from Indonesia and, more recently, brokering a complex peace-sharing agreement in Kenya. He also had a track record of working with Assad.

In his upcoming memoir, Annan says he bore no illusions about Assad’s commitment to peace when he took on his latest assignment but also believed Assad was a “modern man” ready to initiate reforms in Syria.