KYIV, Ukraine—Ukraine began a counteroffensive toward the eastern city of Izyum aimed at disrupting Russian supply lines into the Donbas region, officials said, as Ukrainian forces continued clearing villages north of Kharkiv and President Vladimir Putin warned his Finnish counterpart that joining NATO would risk damaging relations with Moscow.

Putin told Finland’s President Sauli Niinistö in a phone call Saturday that ending its decadeslong nonaligned defense policy by joining the North Atlantic Treaty Organization would be a mistake for Helsinki.

Putin how his invasion of Ukraine had altered the security environment, prompting Finland to seek NATO membership in the coming days.