Three rescue workers died during a second strike on a residential building in a "densely populated district of Kharkiv", Igor Terekhov, the mayor of the eastern city, wrote on Telegram.

Another person died in a drone attack on another building, he said. Police described the fourth victim as a 68-year-old woman and said 11 people had been injured in the night attacks.

Kharkiv governor Oleg Synegubov said Russian forces had launched "at least 15 drones", some of which had been downed.

Ukraine's cities are the target of Russian strikes almost every night, with Kharkiv, near the northeastern border, among those most regularly attacked.

Ukrainian officials have urged the country's allies to supply more anti-aircraft defence systems, in particular modern US-made Patriot systems.

US military aid to Ukraine has been drying up, with a $60-billion funding package currently stalled in Congress.