A suspect in a St. Petersburg cafe blast that killed a Russian military blogger on Sunday has been arrested, Inferfax reported.

The Russian Investigative Committee said on Telegram that Darya Trepova was arrested on suspicion of involvement, the Russian wire service reported.

-ABC News’ Joe Simonetti

Russia plans to move tactical nuclear weapons in Belarus to the country's western borders, Boris Gryzlov, the Russian ambassador to Belarus, said Sunday.

Gryzlov's announcement comes just three days after Russia and the United States clashed in the United Nations over the Kremlin's plans to deploy tactical nuclear weapons in Belarus. U.S. officials denounced the move as a desperate attempt by Russia to avoid military defeat and “threaten the world with nuclear apocalypse."

Gryzlov said in an address aired on the Belarusian STV channel that tactical nuclear weapons "will be moved to the western borders of our Union State and will increase the possibilities for ensuring our security."

The western border of Belarus is shared by Poland, a NATO country supporting Ukraine. Russian forces have used Belarus as a staging ground for the war in Ukraine.

"This will be done despite the noise in Europe and the United States," Gryzlov said.

Ukrainian President Volodymyr Zelenskyy condemned the decision and slammed Belarusian President Alexander Lukashenko, saying he "no longer decides which weapons are on his territory."

"And does (Vladimir) Putin threaten the world? Of course, if Ukraine does not resist, it will fall, Putin will move on, we have emphasized this many times," Zelenskyy said. "With the help of our friends and partners, our army will stand firm and win what is rightfully ours. Victory and our independence."

-ABC News' Will Gretsky

Ukrainian President Volodymyr Zelenskyy condemned a Russian missile strike on Sunday in the eastern Ukraine city of Kostiantynivka that killed six people and injured at least 11.

In an address to his country Sunday night, Zelenskyy called Russia an "evil state" for targeting residential buildings in Kostiantynivka with long-range missiles.

"The evil state must be defeated," Zelenskyy said.

Kostiantynivka, which is close to Bakhmut, is being used as a second-line staging area for Ukrainian troops holding the line on that part of the front.

Zelenskyy said the fighting in Bakhmut was "especially hot" on Sunday.

He predicted a day would come when Ukraine will "celebrate the last Russians being killed or driven out of currently Russian occupied territories, including Donetsk, Luhansk and Kherson Oblasts, as well as Crimea."

-ABC News' Edward Szekeres

A top Russian pro-war blogger has been killed in a bomb attack on a cafe in Russia, according to police.

The explosion on Sunday tore through a cafe in St. Petersburg, killing Vladlen Tatarsky, one of the best-known of the Russian military bloggers who have become influential during the war in Ukraine.

At least 30 other people were injured in the blast, according to the Ministry of Health. Video circulating online appeared to capture the aftermath, showing bloodied people emerging from the heavily damaged cafe.

The Russian Interior Ministry said an explosion has occurred in a cafe on the city's Universitetskaya Embankment.

"One person was killed in the incident, it was military correspondent Vladlen Tatarsky, the Russian Interior Ministry press center told reporters on Sunday.

Denis Pushilin, acting head of the Russian-backed Donetsk People’s Republic, issued a statement describing Tatarsky as "a great patriot" of the Donbass region of eastern Ukraine and Russia. Pushilin blamed the attack on the Kyiv regime, calling it a terrorist regime.

"A man with a difficult fate, Vladlen earned the respect of his comrades-in-arms because he lived and worked for the sake of truth and justice, for the sake of victory," Pushilin said of Tatarsky. "He managed to fight, and in the status of a military correspondent to make his contribution."

Pushilin said Tatarsky was to be awarded a medal "for the liberation of Mariupol" in eastern Ukraine.

It was the most serious bomb attack on a pro-war Russian figure inside Russia since the high-profile assassination of the Daria Dugina, the daughter of the ultra-nationalist Alexander Dugina, who was killed in a car bombing last year.

Tatarsky was a Russian ultra-nationalist and one of the best-known military bloggers, who strongly supported the war in Ukraine. He had also criticized the execution of the war by Russia’s military command.

Tatarsky had become a significant source of information for how the war was being fought on the Russian side.

His killing will likely set off speculation on whether Ukraine or Russia was behind his killing, similar to the Dugina episode.

In the Dugina case, U.S. intelligence sources eventually told The New York Times that Ukraine was behind the attack.

-News Patrick Reevell

The U.S. estimates that Russia has suffered over 100,000 casualties, including over 20,000 killed in action, since it stepped up its winter offensive in December, a White House National Security Council told ABC News.

Half of the 20,000 killed in action were members of the Russian-backed private military Wagner Group, according to White House spokesman John Kirby. The majority of Wagner fighters killed were allegedly ex-convicts, according to Kirby.

Kirby said that the data came from "some downgraded intelligence" that the U.S. has been able to collect. He was unable to provide data on deaths of Ukrainian fighters.

Kirby had earlier told reporters that the Russian casualties took place from battles in Bakhmut since December, however, the National Security Council member said the data encompasses all winter battles.

Many, but not all, of these Russians were injured or killed in the Bakhmut fighting, according to the official.

Kirby emphasized that the U.S. thinks Bakhmut holds "very little strategic value for Russia" and if captured by Russia, it "would absolutely not alter the course of the war in Russia’s favor."

-ABC News' Ben Gittleson