Ukrainian civilians were fleeing heavy fighting on Sunday, as Russia’s armed forces tried to hold off a further dramatic advance by Ukrainian troops in the north-east of the country. Cars packed with families streamed out of the city of Kupiansk, which Ukraine recaptured just over a week ago as part of a stunning counter-offensive. Residents said they had been forced to leave because of heavy day and night shelling. The Russians were bombarding the town and surrounding villages, they said. In the space of a few days Ukraine managed to recapture almost the entire Kharkiv region, liberating at least 300 settlements. Demoralised Russian troops pulled back to a new defensive line on the east bank of the Oskil river, which is about 10 miles from the largely destroyed city of Izium. Others fled across the border back to Russia. Kupiansk, a strategic railway junction, sits on either side of the river. It is on the new frontline after Ukrainian forces on Friday crossed to the right bank. They are now poised to push further into Luhansk province, which the Kremlin and its local proxies have controlled entirely since June, and partly since 2014. Locals in Kupiansk said they had been told to evacuate, and that the explosions were very loud. The city was now without electricity and water and it was difficult to get a proper phone connection, they said, adding that people were hiding in their basements or had taken refuge in their garages. Four medics were killed and two patients injured after Russian forces fired at a psychiatric hospital in the village of Strelechya, the governor of the Kharkiv region, Oleh Syniehubov, said. The facility was in the process of being evacuated and medical staff were removing patients from the hospital when they came under attack. On Sunday, buses were ferrying civilians to the town of Shevchenkove, outside the range of artillery fire. Hundreds of people who had spent six months under occupation queued in the central square to register with the authorities. Others travelled in battered Lada cars and waited by the side of the road at checkpoints. “We spent two days sitting in our cellar. It was impossible to carry on like that so we decided to leave,” said Valery Prihodko. He said he and his relatives had fled Kupiansk and had no clear plans as to where they would go next. “The fighting is pretty bad,” he said. Ukrainian armoured vehicles were visible on the road heading to Kupiansk. The Kraken special forces unit, which was established in Kharkiv in March, said it was in control of the frontline city. It has played an active role in this month’s lightning offensive, in which Ukraine has retaken 300 settlements and an area half the size of Wales. The unit posted a video of dead Russian soldiers killed in fighting. On Saturday it published a second video that showed Russian prisoners repainting a Kupiansk entry sign the blue and yellow colours of the Ukrainian flag. “They are cleaning up their own garbage. What remains is to force these scumbags to rebuild everything they destroyed,” a message read. Russia is now in danger of losing Lysychansk, a key Donbas city it seized over the summer after months of heavy fighting. According to the Institute for the Study of War, the Kremlin has failed to send large-scale reinforcements. It was vulnerable to a Ukrainian counter-offensive, the thinktank said, while carrying out a pointless and “robotic” attack against the city of Bakhmut. President Volodymyr Zelenskiy on Sunday said his armed forces would repeat their success in the Kharkiv region in other occupied parts of the country, including the southern city of Kherson. “Wherever there is Ukraine, there will be our flag. This atrocity – Russian fascism, which repeats what the Nazis did – will remain nowhere,” he wrote on his Telegram channel. Zelenskiy has said that Russia established torture chambers in more than 10 north-eastern areas they occupied. They were located in a police administration basement in the city of Izium, as well as in Kupiansk, and in a railway station in the town of Kozacha Lopan. Russian interrogators delivered electric shocks to victims using a wind-up military field telephone. On Sunday police and forensic investigators continued to dig up bodies from a mass grave in a pine forest on the outskirts of Izium. The dead – 443 of them since February – include more than 20 Ukrainian soldiers. Prosecutors say several had their hands tied together, as well as signs of torture, including broken limbs. The Czech Republic, which holds the EU’s rotating presidency, called for the establishment of an international tribunal for war crimes after the discovery in Izium. “In the 21st century, such attacks against the civilian population are unthinkable and abhorrent,” said the country’s foreign minister, Jan Lipavský. “We must not overlook it. We stand for the punishment of all war criminals,” he added in a message on Twitter. “I call for the speedy establishment of a special international tribunal that will prosecute the crime of aggression.”