The New York Times says US officials have assessed that Ukraine has only enough air defenses to last until next month unless they get resupplied.
The newspaper reported on Friday that Ukraine's air defense systems, crucial to protecting civilians from Russian missiles, are being steadily exhausted by repeated bombardments.
The paper quoted Western officials and analysts as saying, "By next month, Ukraine could struggle to conduct local counterattacks, and by early summer, its military might have difficulty rebuffing Russian assaults."
Ukrainian forces said on Sunday that 45 Russian drones attacked multiple locations, including the Kyiv region and the southern region of Mykolaiv.
The forces said they destroyed 40 of the drones. But local media said fires broke out in Mykolaiv and in the eastern region of Dnipropetrovsk, causing injuries.
Seven people were killed in Russian drone attacks on the eastern city of Kharkiv between Friday and Saturday, which followed large-scale attacks on Kyiv and other parts of Ukraine on Wednesday.