When the Trump administration proposed a peace plan that would recognize Russian rule of the Crimean peninsula, the response from Kyiv was a loud and unequivocal no.

Doing so would violate the nation’s Constitution, President Volodymyr Zelensky of Ukraine told reporters. It would never happen, he declared, not even in exchange for the end of the bloody war raging mostly away from the disputed territory that has been in Russian hands for more than a decade.

Mr. Zelensky’s red line has a hard political reality holding it in place.

Inside Ukraine, formal recognition of Russian control of Crimea would be widely viewed as a dangerous concession to a duplicitous rival and an abandonment of Ukrainians still living in the region. It would also dash hopes for reunification of the families separated by the 2014 occupation — when many pro-Ukrainian residents fled while their elderly or pro-Russian relatives remained behind.

“There is not a single Ukrainian politician who would vote to legalize the occupation of Ukrainian territories,” said Kostyantyn Yeliseyev, former presidential deputy chief of staff. “For members of Parliament, it would be worse than political suicide,” he said.

President Trump expressed bewilderment and frustration at Mr. Zelensky’s reaction on Wednesday, posting on social media that Crimea was “lost years ago” and suggesting that the Ukrainian leader was prolonging the war over a pipe dream.

“He can have Peace or, he can fight for another three years before losing the whole Country,” Mr. Trump wrote.

The seizure of the Crimean peninsula in 2014 began when Russian soldiers — wearing masks and no insignia on their uniforms — seized government buildings and military bases.

The operation was mostly bloodless; Ukrainian soldiers withdrew or switched sides. But that invasion set in motion a Russian effort to capture territory in eastern Ukraine using its army and proxy forces, starting a conflict that killed about 14,000 soldiers and civilians on both sides before Russia’s all-out invasion in 2022 triggered a broader war, according to the United Nations.

That war continues to rage as over the past week the Trump administration has repeatedly threatened to walk away from the peace process. On Thursday, Russian forces launched what appeared to be the deadliest missile and drone attack on the Ukrainian capital since last summer, killing at least nine people and injuring more than 60, according to the Ukrainian authorities.

In peace talks mediated by the United States, Ukraine had hoped to leave control of Crimea out of the discussion. It has sought an immediate cease-fire, freezing the conflict along the existing frontline, as well as security guarantees against renewed attacks, such as the deployment of a European peacekeeping force or eventual membership in NATO.

But the Trump administration rejected that approach this week. Its proposal included an acceptance of Russia’s rule in Crimea and a prohibition on Ukraine joining NATO. In return, hostilities would be halted along the current front lines.

In private conversations, Ukrainian officials have been open to stopping the fighting at the front line. Given Russia’s current momentum on the battlefield, they concede that outcome could favor Ukraine.

More important than where a cease-fire line falls, Ukrainian officials have said, are guarantees that Russia will not use a pause in fighting to regroup and rearm for new attacks. Russia has also warned that Ukraine could use a cease-fire to rearm, but it has largely welcomed the American proposal.

But the peace talks appeared more likely to founder over Crimean recognition than the frontline truce, said Mykhailo Samus, director of the New Geopolitics Research Network, a research institution in Kyiv. “The issue of Crimea is the primary reason for their likely failure,” he said.

Crimea, with a population of about 2 million people, joined the rest of Ukraine in voting for independence from the Soviet Union in 1991. But the region maintained close ties to Russia through its tourism industry, and a majority of the population were Russian speakers. Russian nationalists had claimed the area since soon after the Soviet breakup.

Memories of the annexation are still raw in Ukraine. Recognition of Russian control is also opposed by an organization representing Crimean Tatars, an ethnic group that has deep roots on the peninsula and has faced political retaliation, according to human rights groups.

“Crimea is the homeland of the indigenous Crimean Tatar people and an integral part of Ukraine,” wrote Refat Chubarov, head of the Mejlis, a council of the Crimean Tatars, in a social media post. “No one — under any circumstances — can decide the fate of Crimea except the Ukrainian state and the Crimean Tatar people.”

Among Ukrainian officials, negotiating Crimea’s status is seen as politically risky.

In Kyiv, officials recall that predecessors who signed a lease extension to a Russian naval base on Crimea in 2010, long before the war began, were nonetheless prosecuted later for treason.

And Ukrainians note that recognition would violate principles in post-World War II Europe of opposing the shifting of borders by force.

“No Ukrainian president will ever have the authority to recognize Crimea as it was seized by force as part of Russia,” said Oleksandra Matviichuk, a Ukrainian human rights lawyer who won the Nobel Peace Prize in 2022.

Gauging public opinion inside Crimea is difficult. After Russia’s takeover, many residents voiced support in interviews and posts on social media for joining Russia, but reliable polling is scarce.

The European Union’s top diplomat, Kaja Kallas, has said the bloc opposes formal recognition of Russian sovereignty over Crimea. Turkey has also been a staunch opponent of recognition, in solidarity with the Tatar population and for security concerns about a recognized Russian military presence on the peninsula.

During Mr. Trump’s first term, his administration, too, had issued a formal statement opposing recognition.

The 2018 statement, known as the Crimea Declaration, said the United States would withhold recognition, just as it had of the Soviet occupation of the Baltic States of Latvia, Lithuania and Estonia during the Cold War, a policy that eased those nations’ bids for independence in the late 1980s and early 1990s.

That declaration said that, “the United States reaffirms as policy its refusal to recognize the Kremlin’s claims of sovereignty over territory seized by force in contravention of international law.”

In response to Mr. Trump’s criticism, Mr. Zelensky pointed to the statement in a social media post.

Anna Lukinova contributed reporting from Kyiv.