Fighting around the eastern Ukrainian city of Bakhmut remained "particularly hot", President Volodymyr Zelenskiy said, giving no indication the city had finally fallen to Russia as claimed by the founder of the Wagner mercenary force.
Yevgeny Prigozhin said his troops, involved in a months-long effort to encircle and capture the bombed-out city, had raised a Russian flag on its administrative building.
"From a legal point of view, Bakhmut has been taken. The enemy is concentrated in the western parts," Prigozhin said in video posted on his press service's Telegram account on Sunday.
But there was no indication from Ukrainian officials that Bakhmut, a town of 70,000 before the Russian invasion launched over a year ago, had fallen into Russian hands.
Prigozhin has previously made claims that were premature.
Ukraine's military said in a Monday update Bakhmut and several other towns including Avdiivka were at the "epicentre of hostilities".
"The enemy continues its assault on the city of Bakhmut. However, our defenders courageously hold the city," the military said.
Zelenskiy, in his Sunday night video address, thanked soldiers fighting in Avdiivka, Maryinka, and Bakhmut.
"Especially Bakhmut. It is especially hot there," he said.
Ukrainian Deputy Defence Minister Hanna Maliar had earlier described the Bakhmut situation as "tense". Ukrainian forces were defending their positions and Russian forces were paying scant attention to losses as they attacked, Maliar said.
Reuters could not verify the battlefield reports.
CLASHES IN CITY CENTRE
Ukrainian military commanders have said their own counteroffensive - backed by newly delivered Western tanks and other hardware - is not far off, but have stressed the importance of holding Bakhmut in the meantime.
Ukrainian military analyst Oleh Zhdanov said fighting had engulfed the centre of Bakhmut. Ukrainian forces had repelled 25 enemy attacks, but Russian forces had captured the AZOM metal plant.
"The enemy is attacking the city centre from the north, the east and the south and is trying to take the city under its full control," Zhdanov, who has close ties to the Ukrainian military, said in a video on YouTube.
In Russia, a well-known military blogger, Vladlen Tatarsky, was killed by a bomb in a St Petersburg cafe on Sunday in what appeared to be the second assassination on Russian soil of a figure closely associated with the war in Ukraine.
Russia's state Investigative Committee said it had opened a murder investigation into the blast, which wounded 32 people.
It was not immediately known who was behind the killing. Wagner's Prigozhin said he would "not blame the Kyiv regime" for it but another leading Russian official pointed the finger at Ukraine, without providing evidence.
A Ukrainian presidential adviser said "domestic terrorism" was breaking out in Russia.
NUCLEAR THREAT
Russia invaded Ukraine in February last year in what it calls a "special military operation", claiming Kyiv's ties to the West were a security threat. Tens of thousands of Ukrainian civilians and soldiers on both sides have been killed. Russia has destroyed Ukrainian cities and set millions of people to flight. It claims to have annexed nearly a fifth of Ukraine.
Kyiv and the West call the war an unprovoked assault to subdue an independent country.
In warnings to the West against arming Ukraine, Russian officials increasingly play up the risks of nuclear weapons being used in the war, and last month said they will station tactical nuclear weapons in neighbouring Belarus.
Russia will move those nuclear weapons close to the western borders of Belarus, Russia's envoy to Minsk said on Sunday, placing them at NATO's threshold in a deployment likely to escalate Moscow's standoff with the West.
The weapons "will increase the possibilities to ensure security", Russian ambassador to Belarus, Boris Gryzlov, told Belarusian state television.
"This will be done despite the noise in Europe and the United States."
Russia's arrest of a U.S. journalist is another major diplomatic focus.
U.S. Secretary of State Antony Blinken urged the release of Wall Street Journal reporter Evan Gershkovich in a call on with Russian Foreign Minister Sergei Lavrov, who said Washington should not politicise the case.
Russia's FSB security service said on Thursday it had arrested Gershkovich, accusing him of gathering information about a Russian defence company that was a state secret.
"Secretary Blinken conveyed the United States' grave concern over Russia's unacceptable detention of a U.S. citizen journalist," the U.S. State Department said in a statement that did not mention Gershkovich by name.
Russia's foreign ministry said Lavrov told Blinken that Gershkovich's fate would be determined by a court. He reiterated Russia's assertion, for which it has not presented any evidence, that the journalist was "caught red-handed" last week.
The Wall Street Journal has denied Gershkovich was spying. The White House has called the espionage charge, which carries a jail term of up to 20 years, "ridiculous".
(Reporting by Nick Starkov and Ron Popeski; Additional reporting by Lidia Kelly, Mark Trevelyan and Felix Light; Writing by Lincoln Feast; Editing by Stephen Coates and Robert Birsel)
