Ukrainian forces are continuing to make unexpectedly rapid advances in the north-east of the country, as Kyiv appears to have retaken control of the strategically vital town of Kupiansk and encircled thousands of Russian troops in Izyum, Moscow's stronghold in the north-east sector of the front.
Ukrainian troops have in the past few days pushed Russian forces out of a number of settlements in the region that Moscow occupied since the first days of its invasion.
In a video address late on Friday President Volodymyr Zelenskiy said Ukrainian forces had liberated more than 30 settlements in the Kharkiv region.
Vitaly Ganchev, the head of the Russian-backed administration in the Kharkiv region, said on state television that several villages have come under the control of Ukrainian armed formation and said his administration was trying to "evacuate" civilians from cities including Izyum, Russia's main logistics base in the province, where the British officials described Russian forces as "increasingly isolated".
A local resident of Izyum, who prefers to remain anonymous, told the Guardian that the Ukrainian troops had entered the town of Izyum. Before that, "Russian occupying forces were rapidly withdrawing from the city, leaving ammunition and equipment behind."
The Guardian could not independently verify the situation in the city.
If Ukraine retakes Izyum, it could be its most significant success in pushing back the Russians since the beginning of the invasion.
Izyum has for centuries been regarded as the gateway to the Donbas region of eastern Ukraine and, from there, to the Black Sea. The Russians called the area around it Izyum passage.
Its strategic position has meant Izyum has become a fierce battleground in Russia's invasion of Ukraine, as Moscow used it as a launching point for its assault against Ukrainian forces in Donbas. It fell fully to Russian forces on 1 April, trapping thousands of civilians in a city where as many as 80% of its residential buildings have been destroyed.
"We have managed to cut off the supply lines for the Russian formations in control of the Izium area," said Serhiy Kuzan, a military expert at the Ukrainian Security and Cooperation Center, who has direct knowledge of events in Kharkiv.
Kuzan said the Russian formations in charge of the south-east area of Kharkiv, labelled the Izyum area by military experts, were professional Russian soldiers, not mercenaries or conscripts from Russian-occupied Donbas.
Ukrainian officials shared photos on Saturday showing troops raising the nation's flag over Kupiansk, where rail lines linking Russia to eastern Ukraine converge and which, in the last months, has supplied Russian forces in north-eastern Ukraine.
"Kupiansk is Ukraine. Glory to the armed forces of Ukraine," said Natalia Popova, adviser to the head of the Kharkiv regional council.
The capture of Kupiansk potentially leaves thousands of Russian soldiers trapped at the frontline and cut off from supplies
"The reason why Kupiansk is so important is because it's the major railway junction supplying the Izium formations," said Kuzan. "We saw Russian troops fleeing en masse yesterday. They are now trying to strengthen their positions by dropping reserves in by plane but this is very impractical as the reserves are not prepared," said Kuzan.
The UK's Ministry of Defence has said Ukraine's counter-offensive has taken Russian forces by surprise, adding that Kyiv has advanced 50km (31 miles) along a narrow front line and retaken or surrounded several towns.
"With Ukrainian operations also continuing in Kherson, the Russian defensive front is under pressure on both its northern and southern flanks," reads the statement.
"We are actually surprised by how poorly the Russians have retreated," said Kuzan. "Retreat is part of the art of war. When we retreated, we made sure they suffered losses as they advanced and we did to so to ensure that they only advanced 1, 2 3 kilometres.
"They were so confident that they didn't prepare their defences," said Kuzan. "This has shown that the only advantage they have is in the number of artillery pieces and heavy equipment. So all we need is the same amount."
Following the big territorial gains made this week by Ukraine, Moscow is sending columns of military reinforcements to the Kharkiv region, according to reports in Russian media.
According to the Ukrainian military, Russia is also sending 1,300 Chechen fighters to the southern Kherson region.
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It comes as western intelligence suggests that Russian forces in Kherson are coming under sustained pressure from Ukrainian attacks.
For weeks, Ukrainian officials had telegraphed plans for a planned counterattack in the southern Kherson region, but instead the main focus of this week's counterattack has been Kharkiv in the north-east, taking everyone, including apparently the Russians, by surprise.
Moscow responded by firing rockets into the centre of the city, according to local officials, who said that at least 10 people, including three children, were wounded in an attack that Zelenskiy's chief of staff condemned as revenge for Ukrainian success on the battlefield.
Agence France-Presse, the Associated Press and Reuters contributed to this report