Ukraine acknowledged on Tuesday the difficulties it is facing in its east as Russian forces regrouped after stepping up the pressure and advancing on two cities ahead of a European Union summit this week expected to welcome Kyiv's bid to join the bloc.

The governor of the Luhansk region, the scene of the heaviest Russian onslaught in recent weeks, said Russian forces had launched a massive attack and gained some territory on Monday though it was relatively quiet overnight.

"It's a calm before the storm," the governor, Serhiy Gaidai, said.

Ukrainian President Volodymyr Zelenskyy had predicted Russia would step up attacks ahead of the EU summit on Thursday and Friday. He was defiant in a late Monday address to the nation, though referring to "difficult" fighting in Luhansk for Severodonetsk and its sister city, Lysychansk.

"We are defending Lysychansk, Severodonetsk, this whole area, the most difficult one. We have the most difficult fighting there," he said. "But we have our strong guys and girls there."

Gaidai said Russian forces controlled most of Severodonetsk, apart from the Azot chemical plant, where more than 500 civilians, including 38 children, have been sheltering for weeks. The road connecting Severodonetsk and Lysychansk to the city of Bakhmut was under constant shell fire, he said.

Rodion Miroshnik, ambassador to Russia of the self-styled Luhansk People's Republic, said its 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.

Meanwhile, EU officials said Tuesday that there was no opposition within the 27-nation bloc to granting war-torn Ukraine "candidate status," ahead of a summit expected to green-light the move.

The bloc's executive arm last week proposed taking the symbolic first step to put Ukraine on the yearslong path toward EU membership in a strong sign of support as Kyiv battles Russia's military onslaught.

EU blockade

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 standoff over sanctions imposed over the war in Ukraine.

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 Donbass 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 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 Severodonetsk.

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.

Although fighting has favored 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. Lt. Gen. 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.