Russia summoned the European Union's ambassador in Moscow on Tuesday, fuming over a rail blockade that has halted shipments of many basic goods to a Russian outpost on the Baltic Sea, the latest stand-off over sanctions imposed over the war in Ukraine. On the ground in eastern Ukraine, Russia's separatist proxies said they were advancing towards Kyiv's main battlefield bastion. A Ukrainian official described a lull in fighting there as the "calm before the storm". The latest diplomatic crisis is over the Kaliningrad enclave, a port and surrounding countryside on the Baltic Sea that is home to nearly a million Russians, connected to the rest of Russia by a rail link through EU- and NATO-member Lithuania. In recent days, Lithuania has shut the route for basic goods including construction materials, metals and coal. Vilnius and Brussels say Lithuania is implementing new EU sanctions that came into force on Saturday. Moscow calls the move an illegal blockade and has threatened unspecified retaliation. The EU ambassador in Moscow appeared at the Russian foreign ministry headquarters on Tuesday, Russia's RIA state news agency reported. Overnight, the Kaliningrad governor told Russian television EU ambassador Marcus Ederer was to be summoned and "told of the appropriate conditions involved here". Nikolai Patrushev, secretary of Russia's powerful Security Council, arrived in Kaliningrad to hold a council meeting, RIA reported. Moscow had summoned a Lithuanian diplomat on Monday, but the EU has deflected responsibility from the Lithuanians. Vilnius was "doing nothing else than implementing the guidelines provided by the (European) Commission", said EU foreign policy chief Josep Borrell. HEAVYWEIGHT FIGHT Within Ukraine, the battle for the east has become a brutal war of attrition in recent weeks, with Russia concentrating its overwhelming firepower on a Ukrainian-held pocket of the Donbas region that Moscow claims on behalf of its separatist proxies. Moscow has made slow progress there since April in a relentless fighting that has cost both sides thousands of troops killed, one of the bloodiest land battles in Europe for generations. The fighting has spanned the Siverskyi Donets river that curls through the region, with Russian forces mainly on the east bank and Ukrainian forces mainly on the west, though Ukrainians are still holding out in the east bank city of Sievierodonetsk. In recent days Russia has captured Toshkivka, a small city on the west bank further south, giving it a potential foothold to try to cut off the main Ukrainian bastion at Lysychansk. Rodion Miroshnik, ambassador to Russia of the pro-Moscow separatist self-styled Luhansk People's Republic, said forces were "moving from the south towards Lysychansk" with firefights erupting in a number of towns. "The hours to come should bring considerable changes to the balance of forces in the area," he said on Telegram. The governor of Ukraine's surrounding Luhansk region said Russian forces had gained some territory on Monday. It was relatively quiet overnight, but more attacks were coming, Serhiy Gaidai said: "It's a calm before the storm". Although fighting has favoured Russia in recent weeks because of its huge firepower advantage in artillery, some Western military analysts say Russia's failure to make a major breakthrough so far means time is now on the Ukrainians' side. Moscow is running out of fresh troops, while Ukraine is receiving newer and better equipment from the West, tweeted retired U.S. Lieutenant General Mark Hertling, a former commander of U.S. ground forces in Europe. "It's a heavyweight boxing match. In 2 months of fighting, there has not yet been a knockout blow. It will come, as RU forces become more depleted," Hertling wrote.