The central square of Ukraine's second city, Kharkiv, was shelled on Tuesday by advancing Russian forces who hit the building of the local administration, regional governor Oleg Sinegubov said. 

"This morning the central square of our city and the headquarters of the Kharkiv administration was criminally attacked," Sinegubov said in a video on Telegram. 

"Russian occupiers continue to use heavy weaponry against the civilian population," he said, adding that the number of victims was not yet known. 

He posted footage of the massive blast and debris inside the building.

Kharkiv, a largely Russian-speaking city near the Russian border, has a population of around 1.4 million.

It has been a target for Russian forces since President Vladimir Putin launched an invasion of Ukraine last Thursday. 

Separately, an official in the region of Sumy, which lies north of Kharkiv close to Russia's border, said early on Monday that some 70 Ukrainian soldiers had been killed in Russian shelling on a military facility in the area.

"Many died. Currently, places are being prepared in the cemetery for about 70 dead Ukrainian soldiers," Dmytro Zhyvytsky, the head of the Sumy region, wrote on Telegram after strikes on the town of Okhtyrka.

He posted images of charred buildings with caved-in walls and rescue workers digging through rubble.

The Ukrainian military, however, has not confirmed the deaths.

Russia has denied targeting civilian areas despite rockets landing in residential neighbourhoods.

Ukraine says more than 350 civilians have been killed since Moscow launched the attack last week. 

Meanwhile, it has been reported that a huge Russian military convoy was massing on the outskirts of Ukraine's capital on Tuesday as fears grew the invading forces were set to launch devastating assaults aimed at taking control of Kyiv.