Russian forces have unleashed a barrage of drone attacks on targets in Ukraine. Strikes on the second largest city of Kharkiv have left seven people dead, including three children.

The Ukrainian Air Force said Russia had fired 31 drones into Ukraine's eastern and southern regions from Friday through Saturday.

The Ukrainians say they shot down 23 of the drones. But an oil storage facility in the eastern city of Kharkiv was reportedly damaged, with leaking fuel causing a fire.

A regional governor said more than 10 houses had caught fire, killing seven and injuring three.

In the southern region of Odesa, port infrastructure and nearby buildings were reportedly damaged and four people injured.

Meanwhile, the Russian defense ministry said on Saturday that minister Sergei Shoigu visited a drone manufacturer in central Russia. Officials released footage of the lined-up drones.

The ministry said the company had increased production of unmanned aerial vehicles by 60 percent since Russia's invasion of Ukraine two years ago. It also said the manufacturer is developing a system to automatically identify and attack targets.

Observers say strikes by both sides are likely to intensify, as Ukraine is also bolstering its military drone forces to attack Russia.