WASHINGTON, May 16 (Reuters) - During his first year back in office, U.S. President Donald Trump's blustery negotiating style won him concessions from countries on issues ranging from tariffs to armed conflict.

But with Iran that same brand of coercive diplomacy, marked by public threats, insults and ultimatums, seems to have hit a wall and may be undermining his own efforts to end a war that has shaken the global economy.

With the two sides deadlocked, Trump has signaled growing frustration over the 11-week-old crisis but has shown little inclination to soften his harsh diplomatic approach toward Iran's leaders.

That does not bode well for a quick negotiated settlement, fueling fears that the current standoff - and its unprecedented shock to world energy supplies - could drag on indefinitely with periodic bouts of brinkmanship.

Among the main obstacles, analysts say, is the Iranian rulers' mindset, including their need to save face with their own domestic audience, despite U.S.-Israeli strikes having killed many top leaders and heavily degraded the Islamic Republic's military capabilities.

Though Iran has essentially maintained a chokehold on the vital Strait of Hormuz, giving it considerable leverage, Trump has persisted with a diplomatic playbook characterized by maximalist demands, unpredictability, mixed signals and scathing language.

Even more significant, analysts say, is Trump's insistence on emerging from the conflict framing it as an absolute victory for the U.S. - even if this doesn't match the reality on the ground - while the Iranians must accept total defeat, which they are not likely to do.

"That inevitably gets in the way of reaching a reasonable deal because no government, not just Iran's, can afford to be viewed as having capitulated," said Rob Malley, a former Iran negotiator in the Obama and Biden administrations.

The continuing impasse with Iran comes as Trump faces domestic pressure over high U.S. gasoline prices and his own low approval ratings after he embarked on an unpopular war ahead of November's midterm elections. His Republican Party is struggling to maintain control of Congress.

White House spokeswoman Olivia Wales defended Trump's diplomatic approach based on what she said was a "proven track record of achieving good deals" and insisted that the Iranians were showing increasing "desperation" for an agreement.

"President Trump is a master negotiator who always sets the right tone," she said.

APOCALYPTIC THREAT

His most chilling words came last month when he threatened in a social media post to wipe out Iran's civilization unless it reached a deal - a message that administration officials told the Wall Street Journal was improvisational and not vetted as part of a national security strategy.

Trump ultimately backed down and agreed to a truce. But since his profanity-laced Easter Sunday threat to destroy Iran's bridges and power grid, he has repeated that warning, including to reporters on Air Force One returning from China on Friday.

And last week, Trump told reporters they would know that the current ceasefire had collapsed if they saw "one big glow coming out of Iran," which some interpreted as a threat to deploy nuclear weapons, something he has insisted he would never do.

Trump has reserved some of his toughest words for Iran's leaders, calling them "crazy bastards," "lunatics" and "thugs", and Tehran has responded with its own extensive campaign taunting him with graphic memes and social media posts.

He has repeatedly insisted that Iran has been completely crushed despite evidence to the contrary, said they were "begging" for a deal - only to have the Iranians deny it - while oscillating between demands for "unconditional surrender" and calls for a negotiated settlement. The Iranians, however, have claimed it as a win to have simply survived the military onslaught, showing they can exact a big economic price.

There has been no effort inside the White House to persuade Trump to show greater restraint in his messaging on Iran, according to two sources with knowledge of the matter who spoke on condition of anonymity to discuss internal deliberations.

While opinion polls show his MAGA movement has mostly stood with him, some prominent figures who have supported him in the past have spoken out against the war and criticized his more extreme threats.

SOCIAL MEDIA AFTER MIDNIGHT

Some of Trump's harshest statements, often delivered on his Truth Social platform after midnight, have come at critical junctures, such as last month when he abruptly announced a blockade of Iran's ports and it then retaliated, imperiling the already-fragile ceasefire.

On Monday, Trump dismissed the latest peace proposal from Iranian officials as a "piece of garbage."

"The lack of strategic patience and inconsistency of the president's rhetoric undercuts whatever message he wants to send," said Dennis Ross, a former senior Middle East adviser in Democratic and Republican administrations.

During Trump's visit to Beijing, he mostly refrained from tough verbal attacks on Iran while preoccupied with important relations with China, Tehran's ally and oil customer.

But some analysts suggested it would be best for Trump, who often speaks publicly and gives impromptu phone interviews to reporters, to tone down his rhetoric for good if he is serious about finding an off-ramp in the conflict.

"He talks too much," Iran's Deputy Foreign Minister Saeed Khatibzadeh told reporters last month during a visit to Turkey.

Trump - a former New York real estate developer who touts himself as a master deal maker - has long insisted that being unpredictable is a negotiating tactic aimed at keeping opponents off-balance.

This approach has helped him gain concessions in some cases when he has sought tariff agreements with trading partners, though he has often settled for less than his initial demands. In some conflicts, such as the swift U.S. military campaign against Venezuela that led to the capture of its leader and last year's talks that secured a ceasefire in the Gaza war, his pressure tactics have also yielded results.

With the Iranians, Trump - who campaigned on a promise to keep the U.S. out of foreign wars - wants to look dangerous to intimidate them into giving ground over their nuclear program and other issues, analysts say.

But former U.S. officials who have negotiated with Iran say this is not likely to work, especially given the entrenchment of its clerical and military establishments and the country's pride in its long history.

In fact, Trump's threats may have emboldened Iran's new rulers, considered more hardline than their slain predecessors, who have even less trust for him following U.S. attacks twice in the past year while the two sides were still in negotiations, analysts say.

"There's been this false perception that if you just put enough pressure on Iran, they'll capitulate, but that's just not how it works with Iran," said Nate Swanson, a former State Department official who served on the Iran negotiating team until July.

Barbara Leaf, former Middle East envoy under Trump's predecessor Joe Biden, said that in addition to the president's rhetoric, his Iran campaign has been hindered by "a giddy assumption that Iran was a Venezuela-like problem for resolution (and) wholesale misunderstanding of the regime's inherent resilience."

And some experts believe Trump's approach, which he has said is primarily aimed at ensuring Iran has no path to a nuclear weapon, could backfire.

The U.S. military campaign coupled with Trump's coercive diplomacy might make Iran more - rather than less - likely to ramp up efforts to eventually develop a nuclear bomb so that it might shield itself like nuclear-armed North Korea, analysts say. Iran has long insisted on its right to enrich uranium but says it is only for peaceful purposes.

Adding to the tensions, Trump and the Iranians appear to be operating on different clocks - the impulsive president typically wants a quick deal so he can move on, while Iranian delegations have a history of dragging out talks.

Abdulkhaleq Abdullah, an academic in the United Arab Emirates, a U.S. Gulf ally, said the president could tone down his rhetoric but Iran's intransigence is more to blame for the current stalemate than Trump's "threats and bombastic comments".

Trita Parsi, executive vice president of the Quincy Institute for Responsible Statecraft in Washington, said leaders in Tehran may be interpreting Trump's erratic approach as a sign of desperation and believe they can wait him out.

"In some ways, Trump plays right into their hands," he said.