The front lines in Ukraine had shifted on Sunday as Russia made advances in the fiercely contested eastern Donbas region and Ukraine’s military waged a counteroffensive near the strategic Russian-held city of Izium.

Near the northeastern city of Kharkiv, where Ukrainian forces have been on the attack since early this month, commanders said they believed Russia had been withdrawing troops to reinforce positions around Izium to the south.

Ukraine has scored a series of successes since Russia invaded on February 24, forcing Russia’s commanders to abandon an advance on the capital Kyiv and then making rapid gains to drive them from Kharkiv, Ukraine’s second-biggest city.

Moscow’s invasion, which it calls a “special operation” to disarm Ukraine and protect it from fascists, has jolted European security. Kyiv and its western allies say the fascism assertion is a baseless pretext for an unprovoked war of aggression.

The President of Finland, which shares a 1,300km (800 mile) border with Russia, confirmed on Sunday that his country would apply to join the North Atlantic Treaty Organization (Nato), a major policy shift prompted by Russia’s invasion.

Nato’s deputy secretary-general said he was confident Finland and Sweden, which is also expected to confirm its intention to join, could be swiftly admitted to the alliance, and that concerns raised by Turkey could be overcome.

Since mid-April, Russian forces have focused much of their firepower on trying to capture two provinces known as the Donbas after failing to take Kyiv.

An assessment by British military intelligence issued on Sunday said Russia had lost about a third of the ground combat force deployed in February. Its Donbas offensive had fallen “significantly behind schedule” and was unlikely to make rapid advances during the coming 30 days, the assessment said.

On Saturday night, Ukraine received a morale boost with victory in the Eurovision Song Contest, a triumph seen as sign of the strength of popular support for Ukraine across Europe.

Ukrainian President Volodymyr Zelensky welcomed the win, but said the situation in Donbas remained very difficult and Russian forces were still trying to salvage some kind of victory in a region riven by conflict since 2014.

“They are not stopping their efforts,” he said.

Keeping up pressure on Izium and Russian supply lines will make it harder for Moscow to encircle battle-hardened Ukrainian troops on the eastern front in the Donbas.

Izium straddles the Donets river, about 120km (75 miles) from Kharkiv on the main highway heading southeast.

“The hottest spot remains the Izium direction,” regional governor Oleh Sinegubov said in comments aired on social media.

“Our armed forces have switched to a counteroffensive there. The enemy is retreating on some fronts.”

In Ruska Lozova, a village set in sweeping fields between Kharkiv and Ukraine’s border with Russia, Ukrainian commanders said they believed Moscow was redeploying troops to defend Izium while keeping their opponents pinned down with artillery fire.

“The Russian attack on Kharkiv has been destroyed and they understand this,” said Ihor Obolensky, who commands the National Guard and volunteer force that captured Ruska Lozova eight days ago. “They need to try for a new victory and want to hold Izium.”

Both sides claimed success in military strikes in Donbas.

Russia said on Sunday it had pummelled Ukrainian positions in the east with missiles, targeting command centres and arsenals as its forces seek to encircle Ukrainian units in the battle for Donbas.

But Ukraine’s military also acknowledged setbacks in an update on Sunday morning: “Despite losses, Russian forces continue to advance in the Lyman, Sievierodonetsk, Avdiivka and Kurakhiv areas in the broader Donbas region.”