Russian forces continued their advance on Ukraine’s northern Kharkiv region on Monday, achieving “tactical success”, Kyiv said, as fears grow that Moscow will achieve its biggest breakthrough since the early days of the war. Some of the fiercest fighting is being conducted on the outskirts of the Ukrainian town of Vovchansk, which before the war had a population of 17,000. Ukrainian and Russian reports have confirmed that Russian troops have advanced into the outskirts of the town, where gunfire has been reported in industrial areas. “At the moment, the enemy has tactical success” in the fight for Vovchansk, Ukraine’s general staff admitted in a statement on social media in the early hours of Monday. Ukraine’s general staff said there was also fighting around settlements south of the Pylna settlement. “Our defenders conduct defensive actions [to] inflict damage on the enemy,” it said. In an effort to shift the tide, Kyiv announced on Monday the replacement of the commander overseeing the north-eastern Kharkiv frontline. Ukraine gave no reason for the decision. In a later update on Monday, Ukraine’s general staff announced it was moving additional reserves to the Kharkiv region to try to halt Russia’s advances. Deepstate, a Ukrainian open-source analysis group, earlier said Russia had seized three more villages in the Kharkiv region on Sunday. On Monday Russian troops were advancing near Lyptsi, another small town that is even closer to Kharkiv than Vovchansk. Kharkiv’s governor, Oleh Syniehubov, told local TV: “The enemy is trying to deliberately stretch it [the frontline], attacking in small groups, but in new directions, so to speak.” He said Ukrainian forces were holding Russian troops back but there was a real threat that the fighting could spread to new settlements. Both sides have so far suggested that Russia has not yet been able to achieve a big breakthrough that would allow Moscow to advance towards Kharkiv, Ukraine’s second biggest city. The villages seized by Moscow since Friday lie in what Ukrainian officials call a “grey zone” between the Russian border and Ukraine’s main defensive line. The influential Russian military blogger Rybar wrote on Monday that “no large-scale breakthroughs of the enemy’s defences have been recorded”. Yuriy Butusov, a well-known Ukrainian journalist reporting from the frontlines, said Russian advances had slowed down as their troops took mass casualties. Butusov said the Ukrainian position was improving after the appointment of the new commander. Russia has been gradually intensifying military operations around Kharkiv over the past month, taking advantage of limited Ukrainian air defences to bomb in and around the city in an attempt to persuade more of its 1.3 million residents to flee. Last Friday at dawn, Moscow initiated a fresh unannounced offensive, swiftly seizing nearly a dozen villages and settlements, marking the end of a prolonged stalemate in the Kharkiv region. Russian advances have led to rare public criticism among Ukrainian service personnel, signalling cracks in the morale of the troops. “The first line of fortifications and mines just didn’t exist,” Denys Yaroslavsky, a unit commander fighting in the area, wrote on Facebook in an emotional post condemning Ukraine’s retreat. “The enemy freely entered the grey area, across the border line, which in principle should not have been grey … We come to the conclusion that this is either insane theft or deliberate sabotage,” he said. “I say this because we can die and no one will hear the truth. Then why is it all for?” The public outcry prompted a response from the Ukrainian defence ministry. Oleksandr Yakovets, a senior military official, rejected criticism about the alleged lack of Ukrainian fortifications, calling them “absurd disinformation” that “devalues the efforts of the army engineers”. Yakovets told Ukrainian television that the country’s “best-equipped fortifications” were located further away from the frontlines, along a prepared defensive line. Ukraine is experiencing chronic shortages of manpower and weapons after delays in western funding. As the war nears its 28th month, Ukrainian troops are exhausted, prompting lawmakers last week to approve a controversial bill that enables Kyiv to mobilise convicts, a tactic previously utilised by Russia. Military experts generally agree that the number of Russian forces deployed to the Kharkiv offensive was not enough to capture Kharkiv city. Instead, the attacks could force Ukraine to move up reserves into the region away from defensive positions elsewhere on the frontline. Russian forces are also trying to capture the strategically key eastern city of Chasiv Yar. Its capture would allow Moscow to batter the key remaining Ukrainian-controlled cities in the Donetsk region: Kramatorsk, Sloviansk, Druzhkivka and Kostiantynivka. Moscow may also be trying to minimise Ukraine’s capacity to strike the Belgorod region that lies just across the border. The city of Belgorod, the capital of the region, is just over half an hour’s drive from the border with Ukraine, making it a vital stop on Russian supply lines. The city has come under extensive shelling and drone attacks for months On Sunday, Russian authorities claimed at least 13 people had been killed and 20 injured when a section of an apartment block collapsed in Belgorod. Russian officials blamed Ukrainian shelling for the building’s destruction. Some independent analysts have questioned Russia’s accusations that Ukraine was behind the apartment shelling. Ruslan Leviev, the founder of the Conflict Intelligence Team, an open-source investigation unit, said the incident was probably caused by an errant missile fired by Russia. Moscow’s offensive in Kharkiv comes as Vladimir Putin removed his longtime ally Sergei Shoigu as defence minister in the most significant reshuffle of the military command since the invasion. In a surprise appointment, the Kremlin announced on Sunday night that Shoigu would be replaced by Andrei Belousov, a former deputy prime minister who specialises in economics. The move highlighted the Kremlin’s aim to further ramp up its war economy, allowing Moscow to continue to wage a long war.