NATO's top military officer has said Russia's armed forces are incapable of any major advance. "The Russians don't have the numbers necessary to do a strategic breakthrough," NATO's Supreme Allied Commander Europe Christopher Cavoli told reporters on Thursday. "More to the point, they don't have the skill and the capability to do it; to operate at the scale necessary to exploit any breakthrough to strategic advantage," the general said. His comments come as Ukrainian forces engage in fierce battles with Russia's troops for control of Vovchansk, a key town in Ukraine's northeastern Kharkiv region. Kyiv claimed on Thursday it had checked Russia's advance, but a senior Moscow official said the Russian army had enough resources to keep going. Euronews could not independently verify either claim. Vovchansk, located just 5 kilometers from the Russian border, has been a hotspot in the fighting in recent days. Asked if Russia was about to launch its anticipated summer offensive early, top US and NATO commander, Cavoli said: "We can never be sure." However, he added: "What we don't see is large numbers of reserves being generated some place" needed for such an offensive. Russia began an operation in the Kharkiv region last week, marking its most significant border incursion since the full-scale invasion began in 2022. The move has piled pressure on Ukraine's outnumbered and outgunned forces which are waiting for deliveries of crucial military supplies from the West. Delays in Western assistance have "likely helped" Russia's offensive by forcing Ukrainian forces to conserve material and limiting their ability to defend themselves, according to the US-based Insitute for the Study of War. Ukrainian President Volodymyr Zelensky met top military commanders in Kharkiv on Thursday, saying the region "is generally under control." But he acknowledged on social media that the situation was "extremely difficult". Former Russian defense minister -- now the head of the National Security Council -- Sergei Shoigu insisted Russian troops are pushing the offensive in many directions and that "it's going quite well." "I hope we will keep advancing. We have certain reserves for the purpose, in personnel, equipment and munitions," he said in televised remarks. The Institute for the Study of War calculated that Moscow's army had advanced no more than 8 kilometers from the shared border in Kharkiv. It says Moscow's main aim in the region is to create a "buffer zone" that will prevent Ukrainian cross-border strikes on Russia's neighboring Belgorod region. — Euronews