Ukraine counter-attacked Russian forces on the eastern front on Monday with fighting reported near its second-largest city of Kharkiv, after Western military agencies said Moscow's offensive in the Donbas region had stalled.
Ukrainian interior ministry adviser Vadym Denisenko said in televised comments that fighting near Kharkiv was "our counter-offensive".
"It can no longer be stopped ... Thanks to this, we can go to the rear of the Russian group of forces," he said.
The governor of the Luhansk region in the Donbas, Serhiy Gaidai, said the situation "remains difficult", with Russian forces trying to capture the town of Sieverodonetsk. He said leaders of the Lugansk People's Republic, the territory in Luhansk controlled by Russian-backed separatists, declared a general mobilisation, adding it was "either fight or get shot, there is no other choice".
In the south, fighting was raging around the city of Kherson and Russian missiles struck residential areas of Mykolayiv, the presidential office in Kyiv said.
Reuters was unable to verify the reports.
NATO Secretary-General Jens Stoltenberg said on Sunday Russia's offensive in Donbas had stalled and Ukraine could win the war, an outcome few military analysts predicted when Russia invaded Ukraine on February 24.
Since then, Russian troops have taken heavy losses while blasting cities and towns to rubble, killing thousands and sending more than six million refugees fleeing into neighbouring countries. Russia denies targeting civilians.
In a strategic blow for Russia, which has long opposed NATO expansion, Finland on Sunday confirmed it would apply to join the Atlantic military alliance. Sweden's ruling Social Democrats also backed NATO membership, paving the way for an application and abandoning decades of military nonalignment.
NATO and the United States said they were confident both countries would be accepted into the alliance and that reservations from Turkey, which wants the Nordic countries to halt support for Kurdish militant groups present on their territory, could be overcome.
Ukraine has scored a series of successes since February, reversing a Russian advance on the capital Kyiv and driving Russian forces out of Kharkiv in the east.
Since mid-April, Russian forces have focused much of their firepower on trying to capture two eastern provinces known as the Donbas.
Moscow recognised the independence of the self-proclaimed Donetsk People's Republic and the Lugansk People's Republic in the Donbas days before it launched its invasion.
British military intelligence said Russia had lost about a third of the ground combat force deployed in February, and its Donbas offensive had fallen "significantly behind schedule".
Moscow calls its invasion of Ukraine a "special military operation" to rid the country of fascists, an assertion Kyiv and its Western allies say is a baseless pretext for an unprovoked war.
Fighting around Izium
Ukrainian troops received a morale boost from the country's win in the Eurovision song contest at the weekend, with some saying it was a sign of battlefield victories to come.
"We have shown that we can not only fight, but we can also sing very nice," said Vitaliy, a soldier bunkered down north of Kyiv.
The most intense fighting appeared to be around the eastern Russian-held city of Izium, where Russia said it had struck Ukrainian positions with missiles.
Ukraine’s Joint Forces Task Force said its troops had repelled 17 attacks on Sunday and destroyed 11 pieces of Russian equipment. The command of Ukraine's air force said Ukrainian forces downed two helicopters, two cruise missiles, and seven drones.
Russia continued to target civilian areas along the entire front line in the Luhansk and Donetsk regions, firing at 23 villages and towns, the task force added.
Ukraine's military also acknowledged setbacks, saying Russian forces "continue to advance" in several areas in the Donbas region.
There was also no letup on Sunday in Russia's bombardment of the steelworks 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.
Brightly burning munitions were shown cascading down on the steel works in a video posted by a pro-Russian separatist commander.
Alexander Khodakovsky, a commander of Donetsk, said on his Telegram channel on Monday that 10 Ukrainian fighters emerged from a tunnel at the Azovstal plant holding white flags. Reuters could not verify the report.
Ukrainian President Volodymyr Zelenskyy said "very difficult and delicate negotiations" were going on to save Ukrainians in Mariupol and Azovstal.
Mariupol resident Natalya said her apartment had been bombed and three neighbours had been killed.
"We could not bury them because of the shelling. Each day we've been putting a person into a grave, but we could not cover it up with soil because of the shelling," she said.