YIV, Ukraine—Russia is moving to tighten its hold over occupied parts of Ukraine as its military campaign to take more territory in the eastern Donbas region stalls in the face of fierce resistance, with Ukrainian President Volodymyr Zelensky warning that fighting is set to intensify. In Mariupol, the largest Ukrainian city taken over by Russian forces, Russia published footage of minesweepers preparing to clear the area around the Azovstal steel plant that had for weeks served as a refuge for hundreds of Ukrainian fighters until their surrender earlier this month. Petr Andriushchenko, an adviser to Mariupol’s mayor Vadim Boychenko, said four of the Russian minesweepers had been wounded after a mine exploded on the plant’s territory, with one having sustained serious injuries. Russia didn’t immediately comment on the reports. 

The Mariupol City Council, which functions partly from exile in the city of Zaporizhzhia, itself under threat from Russian forces, Tuesday published the names and photos of nine people it said were collaborating with the Russian occupying forces in Mariupol. “Those collaborators will be punished for the crimes they have committed against their city and country,” it wrote in a Telegram post.

“The Russian occupiers are trying hard to show that they won’t give up parts of Kharkiv and Kherson region and occupied territories of Zaporizhzhia region and Donbas,” Mr. Zelensky said in a late-evening address on Monday. “The coming weeks of the war will be tough, and we should understand this. But we have no alternative but to fight.”

In Russia, meanwhile, one of the few vocal opponents of the invasion in Ukraine, Alexei Navalny, again criticized Russian President Vladimir Putin’s campaign as a court rejected his appeal against a nine-year prison sentence.

“Putin can break a lot of lives, but sooner or later he will be defeated in both this and the stupid war he is waging,” Mr. Navalny said, according to his spokesperson.

From behind bars, Mr. Navalny has called on his supporters to protest the war. He is already serving a prison sentence that began in February last year in relation to a parole violation on an earlier conviction. His latest conviction stemmed from charges of fraud and contempt of court, which, like the other case against him, Mr. Navalny says are politically motivated.

“I am certainly ready to go to jail to tell everyone that people are dying,” said Mr. Navalny, who was speaking via videolink and was repeatedly interrupted by the judge. “No one has killed more Russians than Putin.”