Russian forces on Tuesday pressed forward their weeks-long drive to encircle and capture the eastern Ukrainian city of Bakhmut, where the Ukrainian military described the attacks as constant.
Taking Bakhmut, the scene of some of the year-long war's bloodiest battles, would be Russia's first major prize in more than six months. It would open the way to seizing the last remaining urban centres in the Donetsk region, one of four Moscow claims to have annexed.
"In the Bakhmut sector, the enemy is launching offensive action and is continuously attacking Bakhmut," said the regular evening statement by the General Staff of the Ukrainian Armed Forces.
While most of the Russian attacks were focused on Bakhmut and other towns and villages in Donetsk province, the statement said Russian forces shelled more than 20 settlements in northern regions near the Russian border: Chernihiv, Sumy and Kharkiv.
Ukrainian aircraft launched three strikes on areas of concentration of Russian forces, the statement said.
Reuters was not able to verify battlefield reports.
Russian President Vladimir Putin instructed the FSB security service to bolster security in the four regions Moscow claims to have annexed and also to counter espionage and sabotage operations against Russia by Ukraine and the West.
He was speaking after a Russian regional governor said a drone had crashed near a natural gas distribution station on Tuesday in an apparent failed attack near the town of Kolomna, just 110 km (68 miles) southeast of Moscow.
Ukraine does not publicly claim responsibility for attacks inside Russia. If it was behind the Kolomna incident, it would be its closest attempted drone strike to the Russian capital.
Earlier, Russia's defence ministry also accused Ukraine of launching two attempted drone attacks against two southern Russian regions overnight but said they caused no damage.
'CITY IS ON FIRE'
Around Bakhmut, Russian troops, including mercenary fighters from the Wagner Group, were trying to cut the Ukrainian defenders' supply lines and force them to surrender or withdraw.
"Despite significant losses, the enemy threw in the most prepared assault units of Wagner, who are trying to break through the defences of our troops and surround the city," Colonel General Oleksandr Syrskyi, commander of Ukraine's ground forces, said in a statement.
An unnamed soldier from Ukraine's 93rd Separate Mechanised Brigade, speaking on the Telegram messaging app as explosions boomed in the background, struck a defiant note: "February 28, the town of Bakhmut. The city is on fire, the enemy is pressing. Everything will be Ukraine..."
Russia's state-run RIA news agency released a video clip it said showed Russian Su-25 fighter jets roaring over Bakhmut, which had a pre-war population of around 70,000 but now lies in ruins after months of intense trench warfare.
"We are glad they are ours," says a man in the clip identified as a Wagner fighter, adding the jets helped them "psychologically".
Ukraine's eastern front line resembles "a grinding slog" and Russia is not likely to be able to make significant territorial gains in the near term, the U.S. undersecretary of defense for policy, Colin Kahl, said on Tuesday.
Ukrainian soldiers in the Donetsk region hunkered in muddy trenches after warmer weather thawed the frozen ground.
"Both sides stay in their positions, because as you see, spring means mud. Thus, it is impossible to move forward," said Mykola, 59, commander of a Ukrainian front-line rocket launcher battery, watching a tablet screen for coordinates to fire.
The spring thaw has a history of ruining plans by armies to attack across Ukraine and western Russia, turning roads into rivers and fields into quagmires.
Russia has replenished its forces with hundreds of thousands of conscripts and has intensified attacks all along the eastern front but at a high cost, says Ukraine, which is expected soon to launch its own counter-offensive.
Kremlin spokesman Dmitry Peskov repeated Moscow's stance that it is open to peace negotiations but Kyiv and its Western allies must accept Russia's annexation of Donetsk, Luhansk, Kherson and Zaporizhzhia regions after referendums last September that most governments said were illegal.
"The constitution of the Russian Federation exists, and cannot be ignored. Russia will never be able to compromise on this, these are important realities," Peskov told reporters.
Despite several battlefield setbacks, Russia still controls about a fifth of Ukrainian territory. Kyiv has so far ruled out talks with Moscow and has demanded that Russian troops withdraw to Ukraine's borders in 1991 - the year the Soviet Union collapsed.
On Tuesday, the International Criminal Court's top prosecutor Karim Khan was in Ukraine to investigate Russia's campaign of missile and drone attacks on power and other infrastructure that killed hundreds of civilians and left millions with no electricity or water.
(Additional reporting by Mike Collett-White in Vyshhorod, Ukraine, and Patricia Zengerle and Idrees Ali in Washington; writing by Gareth Jones, Mark Heinrich and Grant McCool; Editing by Nick Macfie, William Maclean and David Gregorio)
