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 Feb 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 1,300 km (800 mile) border with Russia, confirmed on Sunday that his country would apply to join the North Atlantic Treaty Organisation (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. Read full story
'NOWHERE TO BURY ANYONE'
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 120 km (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. Read full story
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."
In western Ukraine near Poland, missiles destroyed military infrastructure overnight and were fired at the Lviv region from the Black Sea, Ukrainian officials said. Read full story
There was also no let-up on Sunday in Russia's bombardment of the steel works in the southern port of Mariupol, where a few hundred Ukrainian fighters are holding out weeks after the city fell into Russian hands, the Ukrainian military said.
Talks were under way to evacuate wounded soldiers from Mariupol in return for the release of Russian prisoners of war, Zelenskiy said.
A large convoy of cars and vans carrying refugees from the ruins of Mariupol arrived in the Ukrainian-controlled city of Zaporizhzhia after nightfall on Saturday after waiting days for Russian troops to allow them to leave. Read full story
Iryna Petrenko, a 63-year-old in the convoy, said she had stayed initially to take care of her 92-year-old mother, who subsequently died.
"We buried her next to her house, because there was nowhere to bury anyone," she said.
Finland and Sweden have both said they see NATO membership as a way of bolstering their security, though Russian President Vladimir Putin has told Finnish President Sauli Niinisto that it would be a mistake for Helsinki to abandon its neutrality.
Sweden's ruling Social Democrats were poised on Sunday to come out in favour of the country joining NATO, paving the way for an application and abandoning decades of military non-alignment. Read full storyRead full story
Germany said on Sunday that it had made preparations for a quick ratification process.
"We must make sure that we will give them security guarantees, there must not be a transition period, a grey zone, where their status is unclear," Foreign Minister Annalena Baerbock said. Read full story
Finland's President Sauli Niinisto said he had been "a bit confused" by the stance of Turkey, which has raised objections to Nordic countries joining and as a NATO member could veto their applications.
"What we need now is a very clear answer, I am prepared to have a new discussion with (Turkish President Tayyip) Erdogan about the problems he has raised," Niinisto said.
As well as losing large numbers of men and much military equipment, Russia has been hit by economic sanctions, while Western states have provided Ukraine with military aid.
Ukrainian Foreign Minister Dmytro Kuleba said he met U.S. Secretary of State Antony Blinken in Berlin on Sunday and that "more weapons and other aid is on the way to Ukraine".