Ukraine's troops are fighting bloody street battles in Severodonetsk, after winning back half of the key city from waves of Russian troops.
Moscow is piling tanks and reinforcements into the crucial town, whose loss would mean Russia would control virtually all of the region of Luhansk.
It comes as Kyiv is in a race-against-time to prepare troops how to use essential British and US-supplied missiles capable of repelling the invasion.
The UK is sending M270 Multi Launch Rocket Systems capable of hitting targets from 50 miles away and the US is sending deadly M-142 missiles.
During a visit close to the Severodonetsk front President Volodymyr Zelensky rallied troops, telling them: “What you all deserve is victory - that is the most important thing.”
It was a rare trip outside the capital Kyiv, which was hit by missiles over the weekend for the first time in many weeks as Russia tries to smash supply lines.
Moscow is desperate to stop the western military supplies which could severely hamper the slow advance its troops are making in the Donbas region.
Ukrainian counter-surveillance teams all over the West are trying to hunt down undercover Russian spies trying to pinpoint how Kyiv is transporting missile systems to the frontline.
Zelensky vowed that his military has “every chance to fight back” after his officials admitted they had lost ground to the Russian advance in Severodonetsk.
Ukraine ’s defence ministry said Russia was throwing troops and weapons into its drive to capture Severodonetsk, the largest remaining Ukrainian-held city in the Donbas region’s Luhansk province.
The Ukraine position in the city is perilous and could become the bloodiest battle scene of the war so far as the Kremlin is suspected of preparing a new offensive from the Black Sea.
Luhansk governor Serhiy Gaidai said: “Our defenders managed to undertake a counter-attack for a certain time, they liberated almost half of the city.
But now the situation has worsened a little for us again.”
The city has become the main target of the Russian grinding offensive in Ukraine’s Donbas region - made up of Luhansk and Donetsk provinces.
New UK missile systems would give the Ukrainians the more precise, long-range firepower needed to reach Russian artillery units, key to Moscow’s battle-plans.
Russian Foreign Minister Sergei Lavrov said Moscow would respond to Western deliveries of long-range weapons to Ukraine by pushing Ukrainian forces further back from Russia’s border.
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Cities in Western Ukraine and Kyiv in the centre are bracing for more long-range missile strikes after Russian President Vladimir Putin said his forces will strike new targets if the West supplied longer-range missiles to Ukraine.
Four large Russian landing ships are sailing towards the Ukrainian coast on the Black Sea from Sevastopol, in the Crimea, which was invaded by Russia in 2014.
It is also suspected Moscow has deployed submarines laden with missiles towards Ukraine in the Black Sea, all of them laden with long-range missiles capable of reaching all over Ukraine.
