Ukraine’s biggest offensive in the war to date has pushed back the front line 70km in a week, claiming the logistical hub of Kupyansk and setting its sights on the military command post of Izyum. The lightning attack in the north-east of the country could lead to the capture of thousands of Russian troops.

“Kupyansk is Ukraine . . . Glory to the armed forces of Ukraine!” Natalia Popova, an adviser to Kupyansk’s city council, wrote on Facebook on Saturday.

The town lies on Russian supply routes to the Donbas region, a swath of largely Russian-occupied territory. Its capture on Friday night has built momentum for Ukrainian forces to encircle and capture a significant pocket of Russian forces further south at Izyum.

“This is a beginning, a good beginning,” said Denys Shmyhal, Ukraine’s prime minister. “Morale is high.”

The success of the counter-offensive has given a tactical and morale boost to Ukrainian forces, and demonstrated their capabilities to western allies.

“Izyum will be under Ukrainian control within hours. Russians escaped and left weapons and ammo behind. City centre is free,” Taras Berezovets, a press officer for the Bohun Brigade of Ukraine’s special forces, said.

Videos posted on social media showed seemingly bedraggled Russian troops who had abandoned their vehicles and positions in a hurry, leaving equipment and food scattered around their positions. Locals cheered on Ukrainian forces as they advanced through liberated villages.

Annalena Baerbock, Germany’s foreign minister, said the Ukrainian advance was a moment of hope. “This is what we need.” Speaking on a visit to Kyiv, she said: “We know that the time between UN gen assembly and Christmas is crucial and weapons support is crucial and we will be at their side.”

The Russian defence ministry issued a rare statement on Saturday acknowledging a pullback from Balakliia and Izyum, but portrayed it as a move intended to focus attention on a different frontline area, rather than a defeat following a massive Ukrainian counter-offensive.

“In order to achieve objectives . . . it was decided to regroup the Russian troops located in the areas of Balakliia and Izyum, to increase efforts in the Donetsk direction,” defence ministry spokesman Igor Konashenkov said.

He said that to this end: “an operation was conducted over three days to wind-down and transfer out” the group of Russian troops located in that area and to move them to the territory of the Donetsk People’s Republic.

Kyiv launched the attack earlier this week, taking advantage of Russian defences that had been depleted after troops were sent south to fend off a Ukrainian offensive around Kherson. As many as 10,000 Russian troops may be caught in the Kharkiv manoeuvre, Lawrence Freedman, emeritus professor of war studies at King’s College London, estimated.

“I am convinced that a few more successes . . . and the Russian troops will flee,” Ukraine’s defence minister Oleksii Reznikov said in an interview published on Saturday with RBK-Ukraine news agency.

“And they will, believe me, because today we are destroying their logistics chains, warehouses and so on . . . It will be like an avalanche, one line of defence will shake and it will fall,” he added.

Nataliya Humenyuk, spokesperson for Ukraine’s armed forces on the southern front near Kherson, said on Saturday that government troops there were also making significant gains. “There is an advance of our troops along the southern front along various sections from two to several dozen kilometres,” she said.

Vladimir Putin, the Russian president, convened a meeting of his security council on Friday, but spokesman Dmitry Peskov said the Kremlin had no comment afterwards and referred all questions to the ministry of defence, Russian news service Tass reported.

Military analysts said Ukraine had launched the two, nearly simultaneous offensives to overwhelm the Russian military’s centralised system of command, which struggles with multidirectional deployments.

“Russian generals are afraid to make mistakes . . . which leads to the centralisation of decision-making, because everybody’s trying to push decisions as much upwards as possible to avoid responsibility. That kills their ability to deal with multidirectional approaches,” said Andriy Zagorodnyuk, a former Ukrainian defence minister.

“So that is exactly what our armed forces are doing . . . attacking where Russians don’t expect and in more than one direction,” he told participants at the Yalta European Strategy conference in Kyiv this weekend.

The success of the Ukrainian offensive to date has led one of the military commentators embedded with Russian troops to describe it as a “catastrophe” and the “biggest Russian military defeat since 1943”.

However, analysts warned of reading too much into early Ukrainian successes, because of the possibility of overstretched supply lines, adding it would be a mistake to underestimate the Russian army’s capabilities.

Below: Counter-offensive on Friday 9 September. Reports on Saturday suggest Ukrainian forces have reached both Kupyansk and Izyum.

To the south, the Kherson offensive is facing stiffer resistance and reportedly taking heavy casualties as its forces confront well-manned and dug-in Russian positions.

“They [the Russians] have very good electronic warfare. They have very good artillery. They do have a few high-tech weapons . . . So, you have got to be careful. You always have to respect the adversary,” General Wesley Clarke, a former supreme Nato commander, said.

Russia is reportedly already sending in more troops. Ukraine’s general staff said that 1,200 Chechen troops had been deployed to reinforce Russian positions around Kherson. Videos posted on social media on Saturday also allegedly showed the Russian army helicoptering in fresh troops to reinforce Izyum.