Moscow-installed officials say Ukrainian shelling killed at least 28 people at a bakery in the Russian-occupied city of Lysychansk.
At least one child was among the dead Saturday, local leader Leonid Pasechnik wrote in a statement on Telegram.
A further 10 people were rescued from under the rubble by emergency services, he said.
Ukrainian officials in Kyiv did not comment on the incident.
Both Moscow and Kyiv have increasingly relied on longer-range attacks this winter amid largely unchanged positions on the 930-mile front line in the nearly 2-year-old war.
However, Ukrainian forces have come under intense Russian attack over the past 24 hours, with continuous assaults along the front line, Ukraine's General Staff said in a statement Sunday.
Fighting has been particularly fierce in the eastern city of Avdiivka, where Moscow is attempting to encircle Kyiv's troops, while Ukrainian forces have also been on the defensive in Kupiansk, Lyman, Bakhmut and Zaporizhzhia, officials said.
One civilian was killed and two injured in a Russian artillery strike in the frontline town of Toretsk, less than 18.6 miles from Bakhmut, said Donetsk regional governor Vadym Filashkin.
The military administration for Ukraine's northern Sumy region said Sunday that Russian forces had shelled the region in 16 separate attacks the previous day, firing on the border communities of Yunakivka, Bilopillia, Krasnopillia, Velyka Pysarivka, and Esman. Gen. Serhii Naiev, commander of the Ukrainian Joint Forces, also said that Kyiv's troops had pushed back Russian sabotage and reconnaissance units attempting to cross the border in the Sumy region.
With Ukraine's soldiers concentrated in the eastern regions of Donetsk, Zaporizhzhia and Kharkiv, the reported incursion suggests that Moscow could be probing vulnerabilities on a new front to further stretch Ukrainian resources.
The United States Congress has been grappling with whether or not to provide additional aid to Ukraine — with the house opting on Sunday to move forward with a $17.6 billion bill that provides military aid to Israel and replenishes U.S. weapons but leaves out Ukraine.
The Senate, however, is slated to vote on a national security package on Wednesday that would send wartime aid to Ukraine, Israel and other allies — a move both Republicans and Democrats said is very much needed.
"There's no surer path to dividing America from our closest allies than by shredding our credibility and abandoning Ukraine,” said Senate Minority Leader Sen. Mitch McConnell.
"While we are respectful of members' schedules and try to limit inconveniences, these challenges at the border and in Ukraine and the Middle East are just too great. And we will need to be here working,” said Senate Majority Leader Sen. Chuck Schumer.
The Senate's push to provide aid to Ukraine comes just days after the European Union signed a deal that will provide roughly $54 billion in aid to Ukraine, which will be paid over the course of four years to help Ukraine stabilize its economy, assist in rebuilding efforts and in positioning it for future European Union membership.
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