KYIV, Ukraine (AP) — Authorities say a nighttime Russian attack using Iranian-designed drones has killed four people and wounded 12 in Kharkiv, Ukraine’s second-largest city. Shahed drones smashed into two apartment buildings in the city near the Russian border that has frequently been targeted during more than two years of war. The Kremlin’s forces in recent months have stepped up their aerial barrages of Ukraine, hitting urban areas and the power grid. The about 1,000-kilometer (620-mile) front line is largely deadlocked, but Kyiv officials say they expect a large-scale Russian offensive in the summer. Officials said three first responders in Kharkiv were killed when Russia struck a multistory building twice in quick succession. Another 14-story building was hit by a drone, killing a woman.