Following an unexpected assault that resulted in the capture of at least nine villages, Ukraine was last night deploying reinforcements to the Russian border.
As frightened people were evacuated under intense bombardment, a Ukrainian official warned that the border towns surrounding Kharkiv's second city were under "round the clock" fire.
Alexander Syrsky, commander of Ukraine’s military, said that the situation had “worsened significantly” in the border area.
“The situation is difficult but the Ukrainian defence forces are doing everything to hold defensive lines and positions and inflict defeat on the enemy,” he said.
Both sides said that they had rushed extra equipment, weapons and soldiers to the area, effectively marking the opening of a new front line in the war.
One Ukrainian soldier confirmed that the Russian attack had come as a surprise.
“There was no first line of defence,” he told the BBC. “We saw it. The Russians just walked in.”
Fighting on the front line has focussed on the Donbas region, around Chasiv Yar and Avdiivka and towards Kupiansk, and analysts were split over Russia’s ambitions with its attack from its Belgorod region towards Kharkiv on Friday.
Some argued that the attack was a diversionary tactic to pull Ukrainian forces away from the main Russian effort in Donbas but others said that Kharkiv may be a target.
The US-based Institute for the Study of War said that the Russian forces did not appear to be large enough to capture the city, although they still posed a significant threat.
“The proximity of Kharkiv City to the border magnifies the significance of limited Russian tactical gains,” it said. “Russian forces do not have to advance much further to begin threatening Kharkiv City with routine shelling.”
Russian forces have enjoyed an advantage in soldier numbers and weapons over the past six months because of a delay in US military aid reaching Ukraine as well as Ukrainian army recruitment problems, and they have intensified their attacks before new US equipment arrives on the battlefield and a fresh mobilisation drive kicks in.
From May 18, new laws will give Ukrainian draft officers more power and the mobilisation age will drop to 25 from 27, which the Ukrainian ministry of defence said would give the army a boost.
“This situation, when some people are fighting at the front lines while others are living their quiet lives, is obviously coming to an end,” Dmitry Lazutkin, a Ukrainian ministry of defence spokesman, told a Ukrainian television station.
In Belgorod, a city in Russia 20 miles north of the border with Ukraine where fighting has focussed, an alleged Ukrainian missile hit a residential block yesterday.
Video showed a gaping hole in a 10-storey residential block and emergency workers clambering over rubble and gingerly picking through the remains of people’s apartments as they looked for survivors.
Russia’s emergency service said that at least 20 people had been injured in the alleged attack and seven people had been killed.
Ukraine-based analysts have said that the bomb may have been accidentally dropped on Belgorod by a Russian warplane aiming for towns in Ukraine.