Ukraine’s government is advising refugees living abroad not to return until the spring amid mounting fears over whether the country’s damaged energy infrastructure can cope with demand this winter. The energy crisis comes as officials in Kyiv warned that the coming winter may herald the heaviest fighting of the war, around the southern city of Kherson where Russian forces have been digging in. With a third of the country’s energy sector compromised by recent Russian missile and drone attacks Ukraine’s deputy prime minister Iryna Vereshchuk warned: “The networks will not cope.” “You see what Russia is doing. We need to survive the winter,” she added. The warning was delivered after a period in which, polling suggested, more Ukrainian refugees had expressed a desire to return home. Vereshchuk said that although she would like Ukrainians to return in the spring, it was important to refrain from returning for now because “the situation will only get worse. If it is possible, stay abroad for the time being.” With no evidence of a letup in the fighting in the country’s east and south, where Ukraine has made recent gains in Russian-occupied areas, many fear the coming winter could be challenging. Ukrainians have already been asked to be sparing in their use of electricity to balance the country’s struggling grid. The prospect of bitter urban fighting for Kherson, the largest city under Russian control, has come closer as Ukraine’s forces have drawn ever closer in their campaign in the south that has seen Russian forces driven back. With Russian-installed authorities encouraging residents to flee to the east bank of the Dnieper River, Oleksiy Arestovych, an adviser to the Ukrainian president, Volodymyr Zelenskiy, said there was no sign Russian forces were preparing to abandon the city. “With Kherson everything is clear. The Russians are replenishing, strengthening their grouping there,” Arestovych said in an online video late on Tuesday. “It means that nobody is preparing to withdraw. On the contrary, the heaviest of battles is going to take place for Kherson.” Of the four provinces the Russian president, Vladimir Putin, proclaimed annexed in September, Kherson is arguably the most strategically important. It controls the only land route to the Crimean peninsula Russia seized in 2014 and the mouth of the Dnieper, the vast river that bisects Ukraine. Russia’s battlefield setbacks have been matched by increasingly febrile rhetoric from Moscow including highly disputed claims that Ukraine plans to use a dirty bomb on its own territory and the bizarre notion by Putin’s security council that Ukraine requires “de-satanisation”. On Wednesday, Putin oversaw Russia’s military rehearsing a response to a nuclear attack in an exercise involving nuclear submarines, strategic bombers and ballistic missiles. The drills, which Putin observed from a control room, were seen as a continuation of Moscow’s unfounded dirty bomb claims. On Tuesday, the Kremlin said it would “vigorously” continue to make the case to the international community that it believed Ukraine intended to detonate a bomb with radioactive contaminants. The Kremlin spokesperson, Dmitry Peskov, told reporters Moscow wanted to prompt an active response from the international community. The Russian defence minister, Sergei Shoigu, on Tuesday made calls to his Indian and Chinese counterparts to convey Moscow’s warning, after a series of calls with Nato defence ministers. In response to the Russian claims, India’s defence minister, Rajnath Singh, said that no side in the war should resort to the nuclear option. “The prospect of the usage of nuclear or radiological weapons goes against the basic tenets of humanity,” Singh told Shoigu, while reiterating the need for an early resolution to the conflict through dialogue and diplomacy. Ukraine and its western allies have rejected Russia’s allegation and voiced concern that Moscow is using it as pretext for a further escalation in the war. Zelenskiy said Russia’s allegation suggested Moscow could be planning to use a tactical nuclear weapon and would seek to blame Kyiv. The US president, Joe Biden, said Russia would be “making an incredibly serious mistake” if it used a tactical nuclear weapon.