KYIV, Ukraine — Russian forces fired drones at two apartment buildings and a power plant in Ukraine’s second-largest city, killing four people, local authorities said Thursday, as the Kremlin’s forces apparently set in motion their strategy for the coming months of war by escalating the bombardment of civilian areas.

Shahed drones smashed into two apartment buildings in Kharkiv, near the Russian border, which has frequently been targeted during more than two years of war. Other drones targeted the power grid.

The Kremlin’s forces in recent months have stepped up their aerial barrages of Ukraine, hitting urban areas. The approximately 1,000-kilometer (620-mile) front line is largely deadlocked, but Kyiv officials say Moscow’s troops have recently been probing for Ukrainian weaknesses on the front line ahead of an expected large-scale Russian offensive in the summer.

The Institute for the Study of War said the goal of Russia’s escalated attacks on civilian areas and the country’s power grid may be to compel Ukraine’s army to deploy air defense systems away from the front line. That would allow Russia to provide more air cover for its ground operations on the battlefield, the think tank said late Wednesday.