Images of the surrendering Ukrainians were publicized by the Russians just as a Russian soldier pleaded guilty in a Ukrainian courtroom to fatally shooting an unarmed civilian, in a widely followed case.

In Brussels, Turkey complicated efforts by NATO to quickly consider membership bids by Sweden and Finland, blocking an initial vote and presenting a list of grievances related to Kurdish groups that it considers terrorists.

While Turkey indicated that it would not ultimately oppose membership for Sweden and Finland, its objections are slowing a process that the West had hoped would quickly strengthen European defenses against further aggression by President Vladimir V. Putin of Russia.

Turkey’s move came against the backdrop of a separate frustration for the West’s challenges to Mr. Putin: Prime Minister Viktor Orban of Hungary, another authoritarian leader, has stalled a proposed European Union embargo of Russian oil.

Ukraine had initially described the mass surrender of the soldiers at Mariupol’s Azovstal steel plant, which its military ordered Monday night, as the only alternative to their near-certain death against hopeless odds, and as a prelude to a prisoner exchange.

But there was no talk from Moscow of swapping any captives, and by Wednesday it was clear that the Kremlin intended to use the prisoners for other purposes.

Russian commentators celebrated the fall of the steel plant and, in particular, the capture of members of the Azov battalion, a Ukrainian regiment with roots as a far-right group, which Mr. Putin has exploited to fictitiously portray the invasion as a battle to rid Ukraine of Nazis.

The Russian Supreme Court said it would hold a hearing next week on whether to declare the Azov group a “terrorist organization,” which could give Moscow cover to deprive the prisoners of rights. Russia has said that 959 soldiers in the plant surrendered, about 800 of them from the Azov battalion. It is believed that up to 1,000 more soldiers remain inside the plant.

Maria V. Zakharova, a Russian Foreign Ministry spokeswoman, said that Azov soldiers had committed war crimes by using kindergartens and medical centers to store ammunition and by using civilians as human shields — accusations that echoed those leveled against Russian troops by the West.

Some of the prisoners were transferred to pretrial detention in the town of Yelenovka, in the Russia-controlled eastern Ukrainian region of Donetsk, Ms. Zakharova said. She accused Ukraine’s forces of having fired rockets at the facility that held them.

Ms. Zakharova said she had no information about a prisoner exchange with Ukraine, and that those requiring medical attention were receiving it. Russia released a video of hospitalized captive soldiers in a separatist-held city east of Mariupol.

Amnesty International urged Russia to respect the rights of the captives, saying they had been “dehumanized by Russian media” and portrayed by Mr. Putin’s propagandists as neo-Nazis, which “raises serious concerns over their fate as prisoners of war.”

Ms. Zakharova said that Russia had encouraged the soldiers to leave the plant for days, and she faulted Ukraine for having waited so long to order them to surrender. “At the moment, the most important thing is that everybody exits,” she said.

Complicating efforts by Ukraine to negotiate a prisoner exchange, the speaker of the Russian Parliament, Vyacheslav Volodin, said lawmakers would consider a ban on “exchanges of Nazi criminals.”

Russia’s move to treat the captives as war criminals came as a Russian soldier pleaded guilty in a Kyiv court to having fatally shot a 62-year-old man on a bicycle — a killing that could be considered a war crime.

Asked by the presiding judge whether he accepted his guilt, the soldier, Sgt. Vadim Shyshimarin, 21, said: “Yes.”

“Fully?” the judge asked. “Yes,” the sergeant replied.

The sergeant had admitted to Ukrainian investigators that he fired the Kalashnikov rifle that had killed the man, Oleksandar Shelipov, prosecutors said.

He told investigators in a videotaped statement that he and four other servicemen had stolen a car at gunpoint and were fleeing Ukrainian forces when they spotted Mr. Shelipov on a bicycle, talking on a phone. Sergeant Shyshimarin said he had been ordered to kill the man so he would not report them.

The sergeant, who is facing 10 to 15 years in prison, was charged under Ukrainian statutes with violating “the laws and customs of war, combined with premeditated murder,” prosecutors said. He was not charged with a war crime under international law.

The trial, part of Ukraine’s effort to document atrocities and identify perpetrators, drew intense interest. On Wednesday, the courtroom and an overflow room were crowded with members of the local and international news media, and the proceedings were broadcast on YouTube.

Legal experts said war crimes prosecutions against senior commanding officers are more difficult and can take far longer because their connections to the crime must be proved in court. In this case, Sergeant Shyshimarin had been accused of actually firing the fatal shot.

The prosecution was extraordinary partly because it proceeded despite its potential to disrupt or even halt future prisoner exchanges between Ukraine and Russia.

“The Russians may now decide to bring cases against Ukrainian P.O.W.s,” said Alex Whiting, a war crimes prosecutor who is a visiting professor at Harvard Law School. “This shows how the atrocity crimes being committed by Russian forces, and Ukraine’s commitment to prosecute them, are so much the center of attention right now.”

The Ukrainian prosecutor in the trial, Andriy Sinyuk, described it as an “unprecedented procedure” in which “a serviceman of a different country is accused of murdering a civilian of Ukraine.”

A Kremlin spokesman, Dmitri S. Peskov, dismissed the proceedings, telling reporters that the accusations leveled against Russian soldiers by Ukraine were “simply fake or staged.”

“We still have no information,” Mr. Peskov said. “And the ability to provide assistance due to the lack of our diplomatic mission there is also very limited.”

Even as Turkey raised concerns about quickly admitting Sweden and Finland to NATO, President Biden on Wednesday formally endorsed both of their applications. He also issued a carefully worded warning to Russia that the United States would help defend both countries while their applications are pending.

In blocking an early procedural vote on the applications, Turkey’s president, Recep Tayyip Erdogan, seemed to be calculating that his cooperation was at a premium at a moment of global crisis. NATO operates by consensus, giving any member political leverage over key decisions.

Soner Cagaptay, director of the Turkish Research Program at the Washington Institute, said Mr. Erdogan was likely angling for concessions before a NATO summit in June, and was likely looking for Sweden to take a stronger stand against Kurdish groups that Turkey regards as linked to the Kurdistan Worker’s Party, or P.K.K., which launched a violent separatist movement in Turkey in the early 1980s.

Mr. Erdogan may also be seeking to unlock sales of American F-16 fighter jets to Turkey, Mr. Cagaptay said.

In an address to lawmakers in Turkey’s Parliament on Wednesday, Mr. Erdogan said the outpouring of support for Ukraine, which he has generally supported, was “bittersweet.”

“Because we, as a NATO ally who struggled with terror for years, whose borders were harassed, big conflicts occurred just next door, have never seen such a picture,” he said.

Turkey’s foreign minister, Mevlut Cavusoglu, signaled that his country would not stop Sweden and Finland from joining NATO and would work to “overcome the differences through dialogue and diplomacy.”

“We understand their security concerns, but Turkey’s security concerns should be also met,” Mr. Cavusoglu told Secretary of State Antony J. Blinken ahead of a meeting at the United Nations in New York.

Valerie Hopkins reported from Kyiv, Neil MacFarquhar from Istanbul, Ivan Nechepurenko from Tbilisi, Georgia, and Michael Levenson from New York. Reporting was contributed by David E. Sanger and Lara Jakes from Washington, Carlotta Gall from Kharkiv, Ukraine, Steven Erlanger from Warsaw and Rick Gladstone from New York.