The Chinese singer stands on a balcony inside a bombed-out theater in the Ukrainian city of Mariupol, the site of a deadly attack last year by Russian forces. Looking at the camera, she sings an excerpt from the Soviet-era patriotic song “Katyusha” and lifts her arms triumphantly into the air.

The video of the singer, Wang Fang, a 38-year-old performer of patriotic songs and Chinese opera, has circulated widely online in recent days, fueling outrage in Ukraine and abroad. She appeared in Mariupol last week as part of a visit by a small group of Chinese media and cultural figures.

The exiled mayor of Mariupol, Vadym Boychenko, said the theater, which was hit by a Russian air attack while civilians sheltered there, was a “symbol of tragedy, a symbol of Russia’s war crimes” that should not be used for entertainment.

“People died there, among them children,” he said in a statement. “To turn the theater into a tourist destination and to sing on the bones of the dead is incredible cynicism and disrespect for the memory of the dead civilians.”

Ukraine’s Foreign Ministry called the performance “an example of complete moral degradation” and said that Ms. Wang and the other Chinese visitors had entered the city illegally.

“Ukraine respects the territorial integrity of China and expects from the Chinese side explanations of the purpose of Chinese citizens’ stay in Mariupol,” Oleg Nikolenko, a spokesman for the ministry, said in a Facebook post.

Chinese officials did not respond to a request for comment.

The episode poses a challenge for Beijing, which has sought to publicly portray itself as a neutral broker since the Russian invasion of Ukraine, while also providing diplomatic, cultural and economic support to Russia, a longtime ally. The video of Ms. Wang and news articles about it were promptly removed from the Chinese internet.

Elizabeth Wishnick, a senior research scientist at the Center for Naval Analyses in Virginia who studies Chinese foreign policy, said the authorities in Beijing might have been concerned about the “tone deafness” of the video.

“It was a shocking display,” she said. “This kind of stunt puts China in a difficult spot and tarnishes its so-called neutral image and its support of sovereignty.”

Mariupol’s Academic Regional Drama Theater was destroyed in 2022 in the midst of the weekslong Russian siege of the city. Hundreds of people had taken shelter in the theater during the siege, and before the attack, the word “children” had been written as a warning in large white letters on the ground outside. At least a dozen people — estimates of the toll vary — died in Mariupol.

After her performance, Ms. Wang and the other Chinese visitors met with Denis Pushilin, the head of the self-declared separatist Donetsk People’s Republic, Mr. Pushilin said in a post on the messaging app Telegram. He praised Ms. Wang’s performance and said the theater was “being restored by St. Petersburg.”

The group also met with the top Russian-installed official in Crimea, Sergei Aksyonov, who said in a Telegram post that they had discussed “cooperation in the field of tourism.” Mr. Aksyonov quoted Russia’s president, Vladimir V. Putin, saying, “China is a brotherly state to Russia.”

Ms. Wang has not commented on the episode. But her husband, Zhou Xiaoping, a prominent nationalist writer who was in Moscow, defended her. In a post that was later deleted, he described Ms. Wang as a “Chinese folk singer without any political identity” and said that she had been pushing for peace.

“I personally applaud my wife for her strength, bravery and determination,” he wrote.

Mr. Zhou, a member of a political advisory group to the Chinese government who has been praised by China’s top leader, Xi Jinping, for exhibiting “positive energy” in his writings, many of which are critical of the United States, said he had not traveled to Mariupol to “avoid unnecessary disputes.”

Ms. Wang, who trained at the Shenyang Conservatory of Music in northeast China, rose to prominence after winning a 2012 competition promoting “red songs,” or Communist patriotic music. She is known for renditions of works like “Hymn to Heroes” and ”Beautiful Chinese Dream,” and she has performed on state television, often in military-style costumes.

“Katyusha,” the song she performed in Mariupol, was a popular patriotic ballad in the Soviet Union during World War II. She sang the Chinese translation of the song, about a woman on a riverbank, singing to a faraway soldier. “Katyusha” is the diminutive form of the name Katherine in Russian; a Soviet rocket launcher was nicknamed for the song.

The international online backlash against Ms. Wang has been heated, with many people saying she should be banned from the stage. The controversy has also affected other performers in the industry.

Another prominent Chinese soprano, Ying Fang, who is a regular at major opera houses, said that she had received racist and violent messages because people had mistaken her for Ms. Wang.

Ms. Fang issued a statement denouncing the messages. “I do not and will not tolerate racism and blind hatred and bullying,” she said.

Amy Chang Chien and Anna Tsybko contributed research.