Ukrainian forces have "stopped" Russia from advancing further into the northeastern Kharkiv region and are now counter-attacking, but Moscow is intensifying its assault on other parts of the front, Ukraine's army said Friday.

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

President Volodymyr Zelensky visited the region's capital on Friday to discuss the battle for Vovchansk, a town that sits fewer than five kilometres (three miles) from the border.

"The Ukrainian Defence Forces have stopped Russian troops in the Kharkiv sector... The situation is under control, counter-offensive actions are underway," the army said.

Despite initial success, "the enemy has got completely bogged down in street battles for Vovchansk and suffered very high losses in assault units", Ukrainian commander-in-chief Oleksandr Syrsky said on social media.

In an attempt to seize the town, Russia "is currently moving reserves from different sectors to support active assault operations, but to no avail", Syrsky added.

He warned, however, that the situation was turbulent on the eastern front, where Russia says its forces have made a string of gains in the past two weeks.

Fighting near the eastern towns of Chasiv Yar, Pokrovsk and Kurakhove has been particularly "intense", he said.

Russia said on Thursday it had made inroads near the city of Bakhmut in Ukraine's eastern region of Donetsk.

Just three days earlier it claimed to have seized the village of Bilogorivka, a key target for Moscow, which is seeking to wrest control of the entire Lugansk region from Kyiv.

Moscow said it launched the offensive in Ukraine's northeast to create a "buffer zone" along the border to prevent future Ukrainian attacks on its territory.

- Railway strikes -

As Ukraine rushed troops to the northeast, Kyiv again accused Moscow of deliberately targeting civilians in strikes.

State-owned train operator Ukrainian Railways reported a flurry of attacks on the Kharkiv region's railway system overnight that damaged tracks, train carriages and buildings.

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

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.

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

More than 11,000 people in the wider region have been evacuated since Russia began its new offensive two weeks ago, according to local governor Oleg Synegubov.

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