KYIV - Ukraine was preparing Monday for a new Russian push in the eastern Donbas region, as Kyiv said its army’s counterattack around Kharkiv had gained momentum.

Since failing to take the capital at the beginning of the invasion in late February, control of Donbas has become one of Moscow’s primary objectives -- but Western intelligence has predicted its campaign will stall amid heavy losses and fierce resistance.

“We are preparing for new attempts by Russia to attack in Donbas, to somehow intensify its movement in the south of Ukraine,” President Volodymyr Zelensky said in his nightly address.

“The occupiers still do not want to admit that they are in a dead-end and their so-called ‘special operation’ has already gone bankrupt,” he added.  Presidential adviser Oleksiy Arestovich told local television that Russian troops were being transferred in the direction of Donbas after withdrawing from Kharkiv following the Ukrainian counteroffensive.

Kyiv’s troops have made so much progress in the northern region that they have almost reached the border with Russia, according to interior ministry adviser Vadim Denisenko, although air raid sirens still sounded in Kharkiv city early Monday.

Arestovich said the retreating Russian forces were being sent towards Lugansk.

“Their task is to take Severodonetsk,” he said. “Well, something is not working for them.”

Severodonetsk is the easternmost city still held by Ukraine, and its fall would grant the Kremlin de facto control of Lugansk, one of two regions -- along with Donetsk -- that comprise Donbas.

But Russia’s attempt to cross a river to encircle it had been repelled with heavy losses of equipment, according to Lugansk governor Sergiy Gaiday.

To further deter the attack, Russian-occupied railway bridges leading to Severodonetsk were blown up, the Ukrainian army said on its Facebook page late Sunday, posting a video of a huge explosion taken from above.

For its part, Russia’s defence ministry claimed it had struck four artillery munitions depots in neighbouring Donetsk.

Air strikes had also destroyed two missile-launching systems and radar, while 15 Ukrainian drones were downed around Donetsk and Lugansk, it added.

But UK defence chiefs said Russia’s offensive in Donbas had “lost momentum”.

Demoralised Russian troops had failed to make substantial gains and Moscow’s battle plan was “significantly behind schedule”, UK Defence Intelligence said.

It added that Russia may have lost one-third of the ground combat forces it committed in February and was “unlikely to dramatically accelerate” its advance in the next 30 days.

Ukrainian commanders say they expect a turning point in their favour by August, but Western powers have cautioned the conflict will turn into a war of attrition stretching into next year.

On Sunday, NATO pledged open-ended military aid for Kyiv, with Germany’s Annalena Baerbock promising it “for as long as Ukraine needs”.

“Ukraine can win this war,” NATO chief Jens Stoltenberg added.

The show of support came as Finland confirmed it would apply to join the alliance, jettisoning decades of military non-alignment.

Just hours after Finland’s announcement, Sweden’s ruling party said it was also in favour of joining, in another remarkable turnaround of political and public opinion.

“The best thing for Sweden’s security is that we apply for membership now and that we do it with Finland,” Social Democratic Prime Minister Magdalena Andersson said.

Stoltenberg said the alliance would look to provide both countries with interim security guarantees while their applications are processed, including possibly by increasing troops in the region.

In Berlin, US Secretary of State Antony Blinken said he heard “almost across-the-board, very strong support” for the bids, despite misgivings from Turkey.

Ankara has accused both Sweden and Finland of harbouring Kurdish extremists, but Stoltenberg said it was not blocking their membership and was confident of finding common ground.

Moscow has repeatedly warned both countries of consequences if they join the alliance, while insisting the Nordic nations have nothing to fear from Russia.

In apparent retaliation, it has pulled the plug on electricity supplies to Finland, with which it shares a 1,300-kilometre (800-mile) border.

On the ground in Ukraine, bitter fighting continued throughout the country.

In Lviv on Sunday, regional governor Maksym Kozytsky said four Russian missiles hit military infrastructure near the border with Poland -- the first time the city has been struck since May 3.

No casualties were reported, and Ukrainian armed forces said they destroyed two cruise missiles over the region.

In the south, the mayor of the southern city of Mykolaiv warned residents that explosions had been heard early Monday.

In the southeastern city of Mariupol, about 600 Ukrainian troops remained holed up in underground tunnels and bunkers under a steelworks there, fighting a rear-guard battle.

In his address, Zelensky said “very complicated and delicate negotiations to save our people” in the vast Azovstal plant continued daily.

Families of the soldiers have appealed to China to intervene to secure the release of the dead and wounded.

The United Nations and Red Cross helped to evacuate women, children and the elderly from the plant where they were sheltering earlier this month.

Petro Andryushchenko, an adviser to Mariupol’s mayor, said on Telegram that a “huge convoy” of 500 to 1,000 cars had arrived in the city of Zaporizhzhia.