Russia’s President Putin visited the occupied port city of Mariupol late Saturday, his first trip to the Ukrainian territory that Moscow illegally annexed in September.

Kremlin spokesman Dmitry Peskov said that Mr. Putin arrived in Mariupol Saturday evening after visiting Crimea, a short distance southwest of Mariupol, to mark the ninth anniversary of the Black Sea peninsula’s annexation from Ukraine. Mariupol became a worldwide symbol of defiance after outgunned and outmanned Ukrainian forces held out in a steel mill there for nearly three months before Moscow finally took control of it in May.

The visits, during which Mr. Putin was shown chatting with local residents in Mariupol and visiting an art school and a children’s center in Crimea, were a show of defiance by the Russian leader two days after a court issued a warrant for his arrest on war crimes charges.

Mr. Putin has not commented on the arrest warrant, which deepened his international isolation despite the unlikelihood of him facing trial anytime soon. The Kremlin has rejected the move by the International Criminal Court as “legally null and void.”

The trip also came ahead of a planned visit to Moscow by Chinese President Xi Jinping this week, expected to provide a major diplomatic boost to Mr. Putin in his confrontation with the West.

Mr. Putin arrived in Mariupol by helicopter and then drove himself around the city’s “memorial sites,” concert hall and coastline, Russian news reports said. The state Rossiya 24 channel on Sunday showed Mr. Putin chatting with locals outside what looked like a newly built residential complex, and being shown around one of the apartments.

Following his trip to Mariupol, Mr. Putin met with Russian military leaders and troops at a command post in Rostov-on-Don, a southern Russian city some 180 kilometers further east, and conferred with General Valery Gerasimov, who is in charge of the Russian military operations in Ukraine.

Mr. Peskov told reporters that the trip had been unannounced, and that Putin intended to “inspect the work of the (command) post in its ordinary mode of operation.”

Speaking to the state RIA agency Sunday, Russian Deputy Prime Minister Marat Khusnullin made clear that Russia was in Mariupol to stay. He said the government hoped to finish the reconstruction of its blasted downtown by the end of the year.

“People have started to return. When they saw that reconstruction is under way, people started actively returning,” Mr. Khusnullin told RIA.

When Moscow fully captured the city in May, an estimated 100,000 people remained, out of a prewar population of 450,000. Many were trapped without food, water, heat or electricity. Relentless bombardment left rows upon rows of shattered or hollowed-out buildings.

Mariupol’s plight first came into international focus with a Russian airstrike on a maternity hospital on March 9 last year, less than two weeks after Russian troops moved into Ukraine. A week later, about 300 people were reported killed in the bombing of a theater that was serving as the city’s largest bomb shelter. Evidence obtained by the Associated Press last spring suggested that the real death toll could be closer to 600.

A small group of Ukrainian fighters held out for 83 days in the sprawling Azovstal steel works in eastern Mariupol before surrendering, their dogged defense tying down Russian forces and coming to symbolize Ukrainian tenacity in the face of Moscow’s aggression.

Russia annexed Crimea from Ukraine in 2014, a move that most of the world denounced as illegal, and moved on last September to officially claim four regions in Ukraine’s south and east as Russian territory, following referendums that Kyiv and the West described as a sham.

The International Criminal Court on Friday accused Mr. Putin of bearing personal responsibility for the abductions of children from Ukraine. United Nations investigators also said there was evidence for the forced transfer of “hundreds” of Ukrainian children to Russia. According to Ukrainian government figures, more than 16,000 children have been deported to Russian-controlled territories or Russia itself, many of them from Mariupol.

Mr. Peskov reaffirmed on Sunday that Moscow considers “any decisions by the International Criminal Court legally null and void.” 

While the move by the ICC Friday was welcomed by Kyiv, the chances of Mr. Putin facing trial are slim because Moscow does not recognize the court’s jurisdiction or extradite its nationals. The United States also is not one of the 123 countries described as “State Parties” to the Court.

In Ukraine, local officials on Sunday reported that at least three civilians had been killed and at least 19 others were wounded by Russian shelling in the previous 24 hours. The three deaths were in the eastern Donetsk region, where fierce battles are taking place for control of the city of Bakhmut, its governor Pavlo Kyrylenko said on Ukrainian TV. Kharkiv regional Governor Oleh Syniehubov said in a Telegram update that a 51-year-old woman was “fighting for her life” after being hit by shrapnel as Russian troops fired on the border town of Dvorichna.

A top Ukrainian presidential aide on Sunday asserted that Ukrainian troops were holding the line near Bakhmut, a key target of Russia’s monthslong grinding offensive. Andriy Yermak, the head of Ukraine’s presidential office, said in televised remarks that “the enemies’ plans to occupy (the city) are now foundering.”

Taking Bakhmut would grant the Kremlin a long-awaited battlefield victory after months of setback, and could pave the way for Russia to threaten other Ukrainian strongholds in the region, including Sloviansk and Kramatorsk.