Western leaders have said the war in Ukraine could last for years and will require long-term military support as Russia brought forward reserve forces in an apparent attempt to capture the eastern city of Sievierodonetsk. “We must prepare for the fact that it could take years,” Nato’s secretary general, Jens Stoltenberg, said in an interview with the German newspaper Bild on Sunday. “We must not let up in supporting Ukraine.” The British prime minister, Boris Johnson, echoed Stoltenberg’s comments. “I am afraid that we need to steel ourselves for a long war,” he said, adding that it was necessary “to enlist time on Ukraine’s side”. It came as the new head of the British army said British troops must prepare “to fight in Europe once again”. “There is now a burning imperative to forge an army capable of fighting alongside our allies and defeating Russia in battle,” Gen Sir Patrick Sanders said, writing to his charges about the challenges they face. The statements suggest the west believes Ukraine cannot achieve a rapid military breakthrough despite the anticipated arrival of fresh Nato-standard arms, while officials in the country have continued to call for rapid help. Ukraine’s forces remain on the defensive in the eastern Donbas region, where fighting continues in Sievierodonestsk. Serhiy Haidai, the governor of the Luhansk region, said Russia was massing forces in an attempt to take full control of the city after weeks of fighting. “Today, tomorrow, or the day after tomorrow, they will throw in all the reserves they have … Because there are so many of them there already, they’re at critical mass,” Haidai told Ukrainian television. Russia already controls most of Sievierodonetsk, Haidai said on Sunday morning, and if Ukrainian forces lose the city, fighting is expected to focus on neighbouring Lysychansk, from which 32 residents have been evacuated over the weekend despite heavy shelling. Russia’s defence ministry also said its Iskander missiles had destroyed weaponry supplied by the west in the Ukraine’s second largest city, Kharkiv, north-west of Luhansk. A Ukrainian interior ministry official said Russian forces were trying to approach Kharkiv, which experienced intense shelling earlier in the war, and turn it into a “frontline city”. The UK’s Ministry of Defence said in a morning update that the intense fighting meant combat units from both sides were “likely experiencing variable morale”, a rare acknowledgment of the pressures faced on both sides. “Ukrainian forces have likely suffered desertions in recent weeks. However, Russian morale highly likely remains especially troubled. Cases of whole Russian units refusing orders and armed standoffs between officers and their troops continue to occur,” the ministry said on Twitter. Ukraine has been calling for a large influx of western weaponry so that it can try to push back the Russian invaders, but what has been offered so far is less than Kyiv has requested. The US, UK and Germany have promised to send 10 rocket artillery systems, but Ukrainian advisers have called for 60 or even 300. One Ukrainian official said that helping the country win a quick victory would be a saving in the long term. Oleksandr Starukh, the governor of the Zaporizhzhia region, said: “We need these weapons because winter is coming,” adding that the country would face greater economic costs if the war dragged on. The problems could extend beyond Ukraine, he said, arguing that Europe could face another wave of immigrants from African and Middle Eastern countries previously reliant on grain exports from Ukraine if the war continued to disrupt maritime exports. Stoltenberg said the price of long-term support for Ukraine was justified, despite the cost of military equipment and rising energy and food prices, because the west would pay a much higher price if Vladimir Putin were to succeed and Russian forces occupied large parts of Ukraine. Johnson, writing in the Sunday Times, said the supply of weapons had to continue, and that it would be necessary to “preserve the viability of the Ukrainian state” by providing financial support “to pay wages, run schools, deliver aid and begin reconstruction”. The German chancellor, Olaf Scholz, conceded in an interview that his country had “concentrated our energy supply too much on Russia” to the point that it was not possible to change course “if the worst came to the worst”. But he defended his predecessor Angela Merkel’s policy of seeking good relations with Moscow. Germany’s minister for economic affairs and climate action, Robert Habeck, said coal-fired power plants would have to be used more as an emergency measure to offset falls in the supply of Russian gas. Bringing back coal-fired power plants was “painful, but it is a sheer necessity”, he said. City mayors and regional governors in Ukraine say that in most cases they already face funding shortfalls and there is no money to repair infrastructure and buildings damaged in places such as Borodianka, north-west of Kyiv because government spending is focused on the war effort. Ukraine’s president, Volodymyr Zelenskiy, visited the south-western frontline at Mykolaiv and the nearby city of Odesa on Saturday. He insisted after his visit that Ukraine would not cede any of the occupied territories in the south of the country to Russia, which occupies the bulk of the country’s coastal areas. “We will not give away the south to anyone. We will return everything that’s ours and the sea will be Ukrainian and safe,” he said. “Russia does not have as many missiles as our people have the desire to live.”