Ukrainian troops are locked in intense battles with the advancing Russian army in two border areas, President Volodymyr Zelensky has said.
It comes as the death toll from a Russian apartment building collapse blamed on Ukrainian shelling rose to 15.
Mr Zelensky said “fierce battles” are taking place near the border in eastern and north-eastern Ukraine as outgunned and outnumbered Ukrainian soldiers try to push back a significant Russian ground offensive.
“Defensive battles are ongoing, fierce battles, on a large part of our border area,” he said in his nightly video address on Sunday.
Ukraine’s general staff said late on Sunday that Russian forces had conducted at least 22 attacks over the previous 24 hours in two parts of the Kharkiv region and had “tactical success”.
The statement did not elaborate.
The Kremlin’s forces are aiming to exploit Ukrainian weaknesses before a big batch of new military aid for Kyiv from the US and European partners arrives on the battlefield in the coming weeks and months, analysts say.
That makes this period a window of opportunity for Moscow and one of the most dangerous for Kyiv in the two-year war, they say.
The new Russian push in the north-eastern Kharkiv region, along with the ongoing drive into the eastern Donetsk region, comes after months when the about 1,000-kilometre (620-mile) front line barely budged.
In the meantime, both sides have used long-range strikes in what largely became a war of attrition.
The Kharkiv incursion may be an attempt to create a “buffer zone” to protect Belgorod, an adjacent Russian border region battered by frequent Ukrainian attacks – to the Kremlin’s embarrassment.
In March, Russia announced plans to evacuate about 9,000 children from the Belgorod region because it was being shelled continuously.
Russian emergency services on Monday finished clearing the rubble in the region’s capital city of Belgorod, where a section of a residential building collapsed following what authorities said was Ukrainian shelling.
Fifteen bodies were pulled from the rubble, Belgorod regional governor Vyacheslav Gladkov said, and 27 other people were wounded.
Another three people in the city of Belgorod were killed by shelling late on Sunday, he said.
Yevgeny Poddubny, a usually well-connected military correspondent for Russia’s state TV corporation VGTRK, said in a recent Telegram post that the Kharkiv assault marked the beginning of “a new phase”.
“We’re pushing the enemy back from the border, destroying the enemy in order to deprive the Kyiv regime of the opportunity to use relatively cheap rockets to attack Belgorod,” he said.
Russian President Vladimir Putin on Sunday replaced Sergei Shoigu as defence minister in a cabinet shake-up.
Mr Shoigu was widely seen as a key figure in Mr Putin’s decision to send Russian troops into Ukraine in February 2022.
Russia had expected the operation to quickly overwhelm Ukraine’s army and for Ukrainians to broadly welcome Russian troops.
Mr Zelensky said fighting in the Donetsk area is “no less intense” than in Kharkiv.
He said the Kremlin aimed to “spread our forces thin” by opening a second active front in Kharkiv.
He described the area around the Pokrovsk region, just inside the Ukrainian border in Donetsk, as “the most difficult”.
Pokrovsk was a town of around 60,000 people before the war and was until recently a two-hour drive from the front line.
Now it is less than half that.
The capture of the Donetsk city of Avdiivka in February opened a door for the Kremlin’s troops to push westward, deeper into Donetsk.
Russia illegally annexed Donetsk and three other regions in 2022 shortly after it invaded Ukraine, and taking control of all of Donetsk is one of the Kremlin’s main war goals.
Though Ukraine was apparently braced for the Russian onslaught in the Kharkiv region, in some areas it only belatedly began preparing fortifications, an issue that has been a source of criticism by Ukrainian troops of their commanders.