The decision by the two Nordic countries to abandon the neutrality they maintained throughout the Cold War would be one of the biggest shifts in European security in decades.

They are the two biggest EU countries that had stayed out of NATO, and Finland's 1300km border will more than double the frontier between the US-led alliance and Russia, putting NATO guards a few hours' drive from the northern outskirts of St Petersburg.

"Finland must apply for NATO membership without delay," President Sauli Niinisto and Prime Minister Sanna Marin said in a joint statement in Helsinki. "We hope that the national steps still needed to make this decision will be taken rapidly within the next few days."

Five diplomats and officials told Reuters that NATO allies expect both countries to be granted membership quickly, paving the way for increased troop presence in the Nordic region to defend them during a one-year ratification period.

The announcement came even as Russia's war in Ukraine was hitting another turning point, with Ukrainian forces driving Russian troops out of the region around the second largest city Kharkiv, their fastest advance since forcing Russia to withdraw from the capital and northeast more than a month ago.

Russian President Vladimir Putin has cited NATO's potential expansion as one of the main reasons for Moscow's "special military operation" in Ukraine launched in February.

Ukraine had also expressed a desire to eventually join the US-led Western alliance, although it has since offered to accept a form of neutral status as part of peace talks.

Moscow has repeatedly warned Finland and Sweden against joining NATO, threatening "serious military and political consequences".

Asked on Wednesday if Finland would provoke Russia by joining NATO, Niinisto said: "My response would be that (Putin) caused this. Look at the mirror."

NATO describes itself as a fundamentally defensive alliance, built around the principle that an attack on one member is an attack on all, effectively granting US allies the protection of American superpower might, including its nuclear arsenal.

On the front lines, Ukraine has mounted a bold counter-offensive in recent days that has ousted Russian forces from villages north and east of Kharkiv, where Russian troops had held the outskirts since the beginning of the invasion.

Reuters journalists have confirmed in recent days that Ukraine is now in control of territory on the banks of the Siverskiy Donets River, around 40km from Kharkiv.

To the north, the Ukrainians have been pushing towards the Russian border. In the latest advance, they announced on Wednesday they had captured the village of Pytomnyk, halfway to the Russian border along the main highway north of Kharkiv.

Ukraine's general staff said in an update overnight: "The enemy is regrouping in order to prevent our forces from advancing further" around Kharkiv, with combat under way where Russians had crossed the Siverskiy Donets.

Ukraine's advances near Kharkiv could put some of Russia's main supply lines to eastern Ukraine, located on the far bank of the Siverskiy Donets, within range of Ukrainian artillery, and even allow it to bombard staging areas inside Russia.

Ukrainian officials reported shelling across the frontier from the Russian border town of Tyotkino, and air strikes that killed and wounded civilians in the northern Ukrainian city of Novhorod-Siversky.

Russia said one person was killed and seven wounded in Solokhi, a village near the border in its Belgorod region.

Elsewhere, the Ukraine general staff said Russia had had some success advancing towards Kudryashivka and Sievierodonetsk, part of Moscow's main assault in the eastern Donbas region.

Air strikes were continuing on Azovstal, a giant steelworks in the ruined southeastern port city of Mariupol where Ukrainian defenders have been making a last stand.

Australian Associated Press