KYIV/SLOVIANSK, Ukraine — Ukrainian forces claimed on Thursday to have pushed forward in intense street fighting in the eastern city of Sievierodonetsk, but said their only hope to turn the tide was more artillery to offset Russia’s massive firepower.

In the south, Ukraine’s defense ministry said it had captured new ground in a counter-attack in Kherson province, targeting the biggest swathe of territory Russia has seized since its invasion in February.