The Kremlin has increased its bombardment across the country, killing at least 40 civilians over the last three days, according to authorities.
Russia is preparing for the next stage of its offensive in Ukraine, a Ukrainian military official said, after the Kremlin said its forces would step up its attacks in "all operational areas".
"It is not only missile strikes from the air and sea," Vadym Skibitskyi, a spokesman for Ukrainian military intelligence, said on Saturday.
"We can see shelling along the entire line of contact, along the entire front line. There is active use of tactical aviation and attack helicopters."
"There is indeed a certain activation of the enemy along the entire front line... Clearly preparations are now underway for the next stage of the offensive,” Skibitskyi said.
The Ukrainian military said Russia appeared to be regrouping units for an offensive towards Sloviansk, a symbolically important city held by Ukraine in the eastern region of Donetsk.
The British defence ministry said on Sunday that Russia was reinforcing its defensive positions across the areas it occupies in southern Ukraine.
The reinforcements include movement of manpower and equipment, defensive stores between Mariupol and Zaporizhzhia, and in Kherson, while Russian forces in Melitopol are also increasing security measures, the ministry wrote on Twitter in a regular bulletin.
Intensified Russian bombardment response to Ukrainian strikes, say authorities
Ukrainian authorities said at least 40 people had been killed in the Russian shelling of urban areas in the last three days, as the war launched by Russian President Vladimir Putin on 24 February intensifies.
Rockets hit the northeastern town of Chuhuiv in Kharkiv region on Friday night, killing three people including a 70-year-old woman and wounding three others, said regional Governor Oleh Synehubov.
"Three people lost their lives, why? What for? Because Putin went mad?" said Raisa Shapoval, 83, a distraught resident sitting in the ruins of her home.
To the south, more than 50 Russian Grad rockets pounded the city of Nikopol on the Dnipro River, killing two people who were found in the rubble, said Governor Valentyn Reznichenko.
Moscow claimed it used high-precision weapons to degrade Ukraine's military infrastructure and protect its own security. It has repeatedly denied targeting civilians.
However, strikes against civilian targets such as the March bombing of a Mariupol theatre, killing an estimated 600 people sheltering on site, and further attacks against the likes of railway stations and shopping centres, as well as atrocities in the Kyiv suburb of Bucha, have remained unaddressed by the Kremlin.
Recently, Ukrainian President Volodymyr Zelensky has called for an international war crimes tribunal to be formed at the Hague in response to the significant loss of civilian life.
Russian Defence Minister Sergei Shoigu ordered military units to intensify operations to prevent Ukrainian strikes on eastern Ukraine and other areas held by Russia, where he said Kyiv could hit civilian infrastructure or residents, according to a statement from the ministry.
His remarks appeared to be a direct response to what Kyiv says is a string of successful strikes carried out on 30 Russian logistics and ammunition hubs, using several multiple launch rocket systems recently supplied by the West.
The strikes are causing havoc with Russian supply lines and have significantly reduced Russia's offensive capability, Ukraine's defence ministry spokesperson said on Friday.