Kyiv has been battling a Russian land assault on its northeastern Kharkiv region since May 10, when thousands of troops stormed the border, making their biggest territorial advances in 18 months.

After initial success, "the enemy has got completely bogged down in street battles for Vovchansk and suffered very high losses in assault units," Syrsky said, but added that fighting near the eastern cities of Chasiv Yar, Pokrovsk and Kurakhove was "intense".

Over 11,000 people have been evacuated from the Kharkiv region since Russia began its new offensive two weeks ago, according to local governor Oleg Synegubov.

Syrsky's comments came as state-owned train operator Ukrainian Railways reported a flurry of strikes on Kharkiv region's railway system, damaging tracks, train carriages and buildings.

"The enemy continues to make deliberate attempts to stop the railway in Kharkiv region," Ukrainian Railways said on Telegram.

"At night, it struck civilian railway infrastructure again. The shelling damaged tracks, buildings, idle freight carriages and an electric train carriage."

The company shared photos showing smoke rising from a wrecked carriage, twisted metal and debris beside tracks and a depot with some blown-out windows.

Long-distance and suburban trains were running as scheduled, the state railway monopoly said, despite repeated Russian strikes on the network, which is vital for both civilians and the military.

Both sides have also accused each other of civilian deaths.

Strikes on the city of Kharkiv, Ukraine's second-largest, killed at least seven people on Thursday, local authorities said.

Separately, Ukraine fired missiles overnight on the Russian-occupied Crimean peninsula, killing two "chance passers-by", the Russian-installed head of Crimea, Sergei Aksyonov, said.