Britain's defence ministry said on Twitter some Ukrainian units had withdrawn, probably to avoid being encircled.
“Russian forces are putting the Lysychansk-Sievierodonetsk pocket under increasing pressure with this creeping advance ... However, its efforts to achieve a deeper encirclement to take western Donetsk Oblast remain stalled,” the ministry said.
On the southern front, Russian forces struck Ukrainian army fuel tanks and military equipment near Mykolaiv with high-precision weapons, Russia's defence ministry said, quoted by the Interfax news agency.
A river port and shipbuilding centre just off the Black Sea, Mykolaiv has been a bastion against Russian efforts to push West towards Ukraine's main port city of Odesa.
Zelenskyy said he had spoken to 11 EU leaders on Wednesday about Ukraine's candidacy and will make more calls on Thursday.
EU CEO Ursula von der Leyen, speaking before the two-day EU summit in Brussels, said: “History is on the march.”
She added: “I am not just talking about Putin's war of aggression. I am talking about the wind of change that again blows across our continent.”
As well as Ukraine, Moldova and Georgia are also seeking to the join the EU in what would be its most ambitious expansion since welcoming East European states after the Cold War.
Russia has long opposed closer links between Ukraine, a fellow former Soviet republic, and Western groupings like the EU and the Nato military alliance.
Diplomats say it will take Ukraine a decade or more to meet the criteria for joining the EU. But EU leaders say the bloc must make a gesture that recognises Ukraine's sacrifice.